All sponges are aquatic and most of them are marine. Freshwater sponges are found in fair numbers—but belonging to relatively few species—in ponds and lakes, streams and rivers, even in lakes formed in the craters of extinct volcanoes 10,000 feet above sea-level. Marine sponges exist in large numbers from the mid-tide level on the shore down to the greatest depths of the oceans. The smallest are about 1 mm. high when fully grown ; the largest are the size of a medium-sized barrel as in Spheciospongia vesparia, the so-called Loggerhead Sponge of the West Indies. On the shore they tend to encrust rocks where, by the coalescence of adjacent growths, certain species may be found in continuous sheets of varying thickness covering areas of several square yards. In the shallow and deep seas the form is more varied, ranging from a crust on stones, shells and dead coral to spherical, finger-shaped, bushy or tree-like, tubular, cup-shaped and funnel-shaped sponges. In general the greater the depth at which they grow the more their form tends to be regular and symmetrical. Those living in sha).low seas are the more colourful, and colour becomes more noticeable as the mean annual temperature of the water increases, as in the tropics, but usually sponges are monochrome with reds, browns, yellows and purples predominating, green less common and usually due to symbiotic algae, and blue very rare and usually the result of symbiotic blue-green algae or bacteria. The texture varies from soft and readily compressible (as in sponges lacking a skeleton, such as Oscarella) to as hard as stone (as in the polyphyletic —i.e. not descended from a common ancestor—group known as the Lithistida). The characteristic feature of a sponge is that it bears one or more usually conspicuous rounded openings. These used to be called oscula (little mouths) but are better described as vents. Under the microscope the rest of the surface is seen to be punctured by minute openings or pores, for which reason sponges are collectively known as the Porifera or pore-bearers. Aristotle was the first to recognise, 2,000 years ago, the animal nature of sponges, and yet it is only within the last 200 years that this verdict has been generally accepted. In the sixteenth century sponges were believed to be solidified sea foam, and in the seventeenth century, it was suggested that they were the homes of marine worms, made by the worms themselves. Otherwise, the general impression was that sponges belonged to the plant kingdom. Not until 1766, when John Ellis dis-covered that they eject currents of water and thus established that they were animals, was Aristotle’s view accepted, and then not by everyone. Even as late as 1841 John Hogg was still arguing before the British Royal Society that sponges were plants. Hogg based his ideas on freshwater sponges. Some of these have long, slender branches springing from a basal crust, and all are green, but turn yellow when growing in places shielded from daylight, just as green plants do. Moreover, in summer they are filled with small brown seed-like bodies. But we now know that the green colour of
Euplectella aspergillum or Venus’s Flower Basket, from 100 fathoms off the Philippines, a hexactinellid (six-rayed) sponge showing the symmetry that is characteristic of deep-sea sponges.
Colour plate PHYLUM CNIDARIA What is commonly thought of as a coral is in fact the calcareous skeleton, or several of these skeletons fused together. secreted by polyps, forms of coelenterates and polyzoans.
The Organ Pipe Coral (Tubipora musica) grows in long, upright, parallel tubes that are united horizontally at intervals by transverse stolons. What is usually seen is the skeleton, as has been explained above, and the red colour is thought to be caused by the presence of iron salts.
The mesozoans are tiny parasitic animalcules found in certain marine invertebrates. There are two orders, the Dicyemida and the Orthonectida, whose simple multicellular structure has provoked much speculation about their place in any systematic classification of the animal kingdom.
In a way still unknown, the worm-like dicyemids infect the kidneys of young squids and octopuses. Typically, each is up to 7 mm. long and consists usually of fewer than 25 elongated cells, the so-called axial cells. Within these, other cells are produced, each of which develops into a new individual before breaking out. This development from unfertilized `agametes’, formed without meiosis, never occurs in the Metazoa. The new individuals remain in the kidney of the host and give rise to a further generation in the same way. When the host reaches sexual maturity, this type of reproduction ceases and the dicyemid differs slightly in appearance and reproduces in a different manner: clusters of cells are produced inside the axial cell, the outer cells in the cluster separating off and dividing to form free-swimming ciliated larvae. These leave the kidney of the host, but whether they infect an intermediate host is not known.
At one stage of their cycle, the orthonectids exist as multinucleate amoeboid plasmodia in the tissues of their hosts, which may be flatworms, nemertine worms, annelids, brittle stars, or bivalve molluscs. The plasmodia reproduce by fragmentation for a time, but eventually, like the dicyemids, they produce agametes, which give rise to distinct sexual forms (male, female, or hermaphrodite) with an outer ciliated cell layer, as in dicyemids, but marked off in rings. An inner mass of cells replaces the single axial cell. Generally, the males are about 0.1 mm. long, and the females two or three times this length. As adults, they leave the host and enter the sea, where they copulate. The resulting fertilized eggs develop into ciliated larvae, which then escape from the parent and infect new hosts. At this point, the larvae lose their outer layer of cells, and the inner cells scatter, each giving rise to a new plasmodium to complete the cycle.
In lacking well-defined ectoderm and endoderm, the mesozoans are clearly outside the main body of multicellular organisms, the Metazoa. They could represent a stage in the evolution of that group from unicellular organisms, in which case the first mesozoans can hardly have inhabited the bodies of invertebrates, as they do now. It has also been suggested that they owe their simple structure and complex life-cycles to their parasitic way of life since these are two common features of parasites. This would make them degenerate forms, and one hypothesis is that they could have evolved from flatworms, although there is little to support this. It is, however, more probable that, like the sponges, they represent an independent line of evolution.
The medusa—represented here by Olindias phosphorica—is a free-swimming coelenterate form with a characteristic radial canal system and bell formation. It is essentially a free-living organism as opposed to the polyp, the other highly contrasting mode of life among the coelenterates.
Members of this class are mostly free-living and have a more highly developed AZM than the primitive hymenostomes. The AZM winds clockwise to the cytostome. Many of the more advanced spirotrichs have no somatic cilia but this loss is secondary. There is a wide variety of structure which prevents generalizations on morphology; most of the orders, however, are relatively compact.
Heterotrichs (Order Heterotrichida)
The somatic cilia are more or less well represented and of uniform size. The oral cilia are several times larger than the somatic cilia and may be fused into cirri. Various habitats are used: Clevelandella and Melanesia are both invertebrate gut symbionts, Spirostomum is found in freshwater and is highly motile, and Stentor is found in freshwater but is sessile. Spirostomum and Stentor are both large and maybe three mm. long. Both are highly contractile and can shorten to less than one-quarter of the original length. Spirostomum has its cytostome situated about two-thirds of the way down the body and is fed by particles passing down the oral groove.
The latter is wound spirally around the animal for between half and one turn. A contractile vacuole is in the posterior and from it a single tubule for drainage of the cytoplasm extends nearly the whole length of the organism. Spirostomum feeds on bacteria and other protozoans. Food vacuoles take a regular course, first passing forward to the anterior tip and then journeying to the posteriorly situated permanent cytoproct.
Stentor is sessile when feeding, being attached by a ‘foot’ at the thin end of the conical body. The base of the cone carries the oral cilia (AZM) as a single row on the rim. Only at the buccal region is there an indentation and the food particles are led to the cytostome. The macronucleus is in the form of a string of beads with up to twenty segments. There are many micronuclei.
Hypotrichs (Order Hypotrichida)
Examples of this order are among the most active and commonly found ciliates. They are generally of medium size (70-l50µ long) and occur in mud and rotting vegetation. There are no generally distributed somatic cilia. These are replaced by bristle-like cirri composed of fused cilia on the dorsal surface. The distribution of cirri is a factor used in the separation of several families and genera. The AZM is the main feature of the oral ciliature.
Euplotes and Stylonichia are good examples of the order. In Euplotes the macronucleus is horseshoe-shaped. Adjacent to the center of the macronucleus and on its outer surface is a single micronucleus. The dorsal cirri are reduced to only eighteen bristles. Some of the posterior cirri are connected by a system of subpellicular fibrils to a centre, the motorium, at the anterior of the cell. The other cirri have several fibrils running from their bases at different angles. Experiments in which some fibrils are cut suggest that they serve to co-ordinate the posterior cirri.
Entodiniomorphs (Order Entodiniomorphida)
Structurally these are the most complex of the ciliates and therefore of the whole phylum. Somatic cilia are absent, and tufts of cirri are confined to a maximum of three regions.
The posterior is sometimes produced into spines. They are found in the rumen and reticulum portions of the stomach of ruminants, where they always occur in great num-ben. Being extremely specialised in this bizarre environment with its continuous comings and goings of food materials, low pH, and the near absence of oxygen, the order is, not surprisingly, most difficult to culture. In consequence little is known about them except their morphology.
The question of importance is how does the presence of perhaps 101° 100g long ciliates affect the nutrition of a cow. It was once thought that the ciliates converted cellulose into digestible carbohydrates, which would be of considerable benefit to the host. Later work has shown that cattle thrive every bit as well when their rumens have been sterilized of ciliates.
More recently it has been claimed that the frequently fatal disease, bloat, is caused by cattle feeding on too rich pasture, which in turn provided food for and produced many bacteria which were ingested too readily by the ciliates. The overfed ciliates bunt in the rumen, liberating their cell proteins which stabilize the foam of normally produced gases. The foam cannot escape and the rumen swells.
These protozoans have many cilia and nuclei of two sizes. Because the mastigophoran Opalina also has many flagella (or cilia), it is the nuclei that act as the criterion and establish the true ciliates as a distinct group. Other features include a kinety system of fibrils and granules for co-ordinating the cilia, characteristic cross-wise fission and a characteristic form of sexual reproduction called conjuga-don.
The range of form exhibited in this class is so great that it suggests a very long independent evolutionary history. While it is relatively easy to place a protozoan as a ciliate it is often quite difficult to determine its order unless the genus is already known, because elaborations and fusion of some cilia, often accompanied by loss of others, has occurred in each order. This type of parallel evolution produces genera that are superficially similar.
The only sure method of identification is the application of one of the silver line staining techniques, in which it is possible to show portions of the kinety system and so obtain the necessary characterization upon which their taxonomy is based. But this is a method not readily or easily performed.
The habitats are various: parasitic, commensal and free-living species are found in many orders. The degree of morphological specialization in some of the spirotrichs is noteworthy as an example of elaboration in a single cell: adaptation to unusual habitats such as the interstitial spaces in sand has lead to great elongation of up to 5 mm. in some species, while some free-living freshwater species are only 15μ long.
Holotrichs (Subclass Holotricha)
This group is regarded as more primitive than the other subclasses for there is lack of development in the peristomial membranelles. A feature that cuts across all other complications and simplifications of the structure is the way the cilia are fused to form accessory organelles in the funnel-shaped cyto-pharynx, or cell mouth. It is characteristic of this subclass that no fused structures are present and the mouth is plainly open. Typically the body (somatic) cilia are of about the same length and present all over the cell.
Gymnostomes (Order Gymnostomatida) As the name suggests the mouth is without ciliary organelles. This is a most diverse group of structurally simple ciliates. But diversity has led to considerable changes in the classification in recent years. A comparison of current classifications shows that no general agreement has yet been reached. Most species in the order contain one meganucleus and one or more micronuclei. The mouth may be anterior and terminal or lateral. If the mouth is lateral then the body is produced into a thin extension to form the new anterior end.
This is the basis for the classification of the suborder Rhabdo-phorina, in which the cytopharynx (the space immediately within the cell opening, the cytostome) is extensible. The other suborder is the Cyrtophorina which has a rigid inextensible cytopharynx.
Suborder Rhabdophorina In this group are found some remarkably voracious feeders, carnivorous in every sense of the word despite their smallish size. A simple form like Holophrya is oval and about 160μ long. It is found on the surface of freshwater where it feeds on dino-flagellates.
The remains of past meals are clearly visible in several food vacuoles in the cytoplasm. A contractile vacuole partly maintains the water balance in the cell. Didinium is similar in shape but has much reduced ciliary fields. Only two bands are left, one round the mouth and one near the equator. This organism can ingest a whole Paramecium although the prey is several times larger than itself.
The vigour with which the prey is attacked, apparently paralysed and then drawn into a mouth which seems far too small is fascinating to watch. Other genera include Dileptus and Lionotus which have extended trichocyst and cilia-bearing anterior processes giving a comb-like appearance to the region in front of the mouth. This shape is an adaptation to living between sand grains and environments where free swimming is impossible.
The higher ciliates have a food-gathering ring of cirri, known as the adoral wreath, around the gullet, and in this photograph it can be clearly seen, together with the nucleus and food vacuoles ( x 1,680).
The ventrally placed mouth is believed to be of evolutionary significance in view of the fact that the majority of ciliates are like this but have additional elaborations. This is a small group. A typical genus is Chilodonella. The ventral mouth is visible from the dorsal side through the cell. The anterior process is considerably thinner than the main posterior part. A line of cilia, the dorsal brush, runs from the edge of the cell to a region over the mouth, and should not be confused with the feeding organelles in other ciliates which sweep food into the mouth.
Suctorians (Order Suctorida)
The suctorians are a compact group only recently considered as holotrichs. The adult form is sessile and without cilia. Food is captured by long knob-ended tentacles which are capable of paralysing prey.
After capture the prey is not drawn to the body, for there is no mouth, but is sucked dry by the tentacles. Recent film studies show the steady flow of cytoplasm from an organism such as Tetrahymena into a suctorian. Reproduction is by several methods.
The commonest is internal budding. After division of the nucleus the anterior pellicle sinks into the cytoplasm in a circle and a young ciliated suctorian develops around the new nucleus. When mature the daughter cell leaves the parent and swims freely. Later it settles down on some object, secretes a non-contractile stalk and loses its cilia to become a small adult. Stained preparations of suctorians show that all stages of development
the kinety system is present even when cilia are not. Typical genera are Tokophyra, Podophyra and Adult:.
Trichostomes (Order Trichostomatida) This group has many features in common with the gymnostomes but is distinguished from them by the presence of a vestibulum. This organ is a depression of the pellicle complete with the cilia which forms a tube leading to the mouth. The cilia of the vesti-bulum urge food into the mouth.
The cytoplasm adjoining the mouth appears to be highly vacuo-lated, and algae or bacteria upon which a free-living genus, Coelosomides, feeds, are digested here. The structure of the gut parasite of amphibians and mammals, Balantidium colt, superficially resembles some hymenostomes or heterotrichs.
The mouth, however, is fed by vestibular cilia which are derived from and develop in a way similar to the normal somatic cilia covering the cell. A common freshwater genus is Colpoda, characteristically kidney-shaped and about 75μ long. The mouth is on the indented side, and the vestibular cilia, clearly visible in stained preparations, are a little shorter than the somatic cilia.
Hymenostomes (Order Hymenostomatida)
This order marks the starting point for the elaborations found in the remaining ciliates. The feeding apparatus is described in some detail here and the names of structures are applicable later. The first of these structures is the buccal cavity between the cytostome and the buccal overture, which is a line of demarcation between the somatic cilia and the buccal cilia.
The buccal cavity has two types of ciliary organelles: the adoral zone of membranellae (AZM) on the left of the cytostome and the undulating membrane (UM) on the right. (Left and right are used for the organism whose mouth is considered to be ventral when the observer is looking from the dorsal surface to the ventral, as left and right are on our own bodies.
In order to get clear pictures and because of the standard way microscopical preparations are made the organisms are always viewed from the ventral surface. So, in drawings and photographs, left and right are trans-posed, as when two people look attach other face to face.)
The UM is a row of cilia that are fused together and beat in co-ordination. The AZM is variable and may be a row of fused cilia as in the spirotrichs or may be divided into several sheets of fused cilia as in Tetrahymena. The buccal cavity has no cilia except for those contained in the UM and the AZM.
The oral ciliature is developed into two fields of cilia in the buccil cavity which opens into a vesti-bulum lying in a shallow oral groove. Paramecium feeds largely on bacteria which are gathered by a feeding current created by cilia in the oral groove. The particles are swept down this groove into the shallow vestibulum and, if accepted into the buccal cavity, to the cytostome. Particles can be rejected, in which case they are swept across the buccal cavity at right angles to the normal ciliary beat. Movement in Paramecium is, as in most ciliates, by means of the somatic cilia covering the body. The cilia beat in a co-ordinated pattern to give waves of propulsion running down the surface.
The cilia lie in rows called meridians, running in the antero-posterior direction. Each row or kinety has cilia connected by fibrils running under the pellicle. The kinetics are cross-connected by transverse fibrils at a level deeper than the longitudinal fibrils. The whole of these interconnections beneath the pellicle is known as the infra-ciliature.
Paramecium is well known for its avoidance reaction. The normal course of swimming is forwards and rotating about its own long axis. But when an unpleasant stimulus is reached the organism stops, reverses at a slight angle for two to four lengths and then moves forward again at a slight angle to the reverse direc-tion. The net result is that the organism turns from its original course and heads away from the unpleasant stimulus. It appears not to feed when moving quickly.
while the macronucleus divides by splitting without the characteristic mitotic apparatus. The methods of sexual reproduction in the ciliates are conjuga-tion and autogamy. In conjugation two individuals come together and join at their oral surfaces. Both macronuclei partially degenerate by enlarging and becoming diffuse and then drawing out into strands which later break down to give numerous weakly staining remnants of the original nuclei. The micronuclei, of which Paramecium aurelia has two, divide meiotically, giving eight daughter nuclei. Seven of the eight degenerate and the remaining one divides again to produce two homozygous gametic nuclei.
One gametic nucleus from each partner migrates into the other partner and fuses with the stationary gametic nucleus. The zygotic nucleus divides twice and while these divisions are occurring the con-jugants separate. Of the four resultant micronuclei two become macronuclei and two micronuclei.
The latter divide again and the exconjugant divides, one macronucleus going to one daughter while the other goes to the .second. Each daughter thus receives two mitotically derived micronuclei and one whole macronucleus.
The second form of sexual reproduction is autogamy. Here a single organism goes through the motions of conjugation without another conjugant. Two homozygous gametic nuclei are formed and these fuse to form a zygote. Thereafter divisions and a new macronucleus are formed as in conjugation. Genetically autogamy brings about recombination of genetic material in a wasteful manner as the genes contained in the daughter micronuclei are lost for ever. The process is similar to the endo-meiosis recently reported to occur in aphids. Recent genetical studies have shown that Paramecium species have classical Mendelian inheritance for many characters.
Although cytological studies . have suggested the above description of sexual reproduction for some eighty years it was only from genetical analysis that certain divisions could be more surely determined as meiotic. Further, some characters were found to be inconstant, in particular the properties of the surface protein of cilia. These proteins developed in response to the temperature of the environment. Whether the change in protein structure is accidental or has a function is not known, for each variation is a newly discovered phenomenon even in parasitic protozoans.
Peritrichs (Order Peritrichida)
These include the sessile ciliates (Sessilina), which live on contractile stalks and are common near the surface of overgrown aquaria, and motile forms (Mobilina) which are believed to be derived from the sessile forms. The characteristics of the order are three specialised rows of cilia around the mouth and the general absence of somatic cilia. Viewed from the anterior the oral cilia are seen to wind counter-clockwise into the mouth. Vorticella is a typical and common genus. The body is bell shaped and mounted with the pointed end on a contractile stalk. During feeding the stalk is elongated and the crown of peristomial cilia expanded.
Paramecium is an example of Ciliates (Class Ciliata)
This class contains two orders, Heliozoa and Radiolaria, which are superficially rather similar. Individuals of both orders may have siliceous skeletons arranged as spheres with holes in them. Axopodia, pseudopodia stiffened with axial filaments, project through the holes. Food is caught on the axopodia and is drawn by the movement of the cytoplasm to the cell body where it is ingested. The radial arrangement of the axopodia is the same from whichever side the organism is viewed.
Locomotion using the tips of the axopodia is slow; in both orders water currents and sometimes a flotation mechanism help dispersion.
Heliozoans (Order Heliozoa) The “sun animalcules”, beloved of Victorian microscopists, are freshwater organisms found in peat and other acid pools. The various species are generally more than 40μ in diameter and as much as I mm. The cytoplasm of Actinosphaerium is clearly divided into two concentric spheres. The outer is vacuolated and considered to be ectoplasm. The inner sphere is dense and contains the food vacuoles and most other organelles.
The roots of the axial filaments arise in the dense cytoplasm. This and the related genus Actinophrys are both naked, that is, without a stiffening skeleton. Raphidiophrys and Clathrulina have siliceous skeletons. In the genera studied in detail, cysts form in well-fed individuals. Within the cyst, gametes are formed and these fuse with each other. Hatching of the zygote seems to require winter conditions. Repro-duction is also by repeated binary fission.
Radiolarians (Order Radiolaria)
These arcs all marine and mostly pelagic. Structurally they are separated from the heliozoans by having a distinct membrane between the ectoplasm and endoplasm. The ectoplasm in some species appears to be capable of secreting freshwater into vacuoles. The presence of such vacuoles clearly lightens a skeleton-bearing animal and enables it to rise to the surface of the sea. Not all radiolarians have massive skeletons.
The suborder Actipylina has radial spicules and some members of the suborder Peripylaria have four-pronged spicules or none at all. Silica is the principal skeletal constituent. The best-known radiolarians are those with full and persistent skeletons, which sink to the bottom of the ocean to form a thick ‘radiolarian ooze’. The variety and detail make these remains beautiful objects for microscopy.
Heliozoa is an example of Class Actinopoda
Binary fission is shown in several stages. After the nucleus has divided, the cytoplasm becomes narrowed, separation continues but still, the cells are joined by a thick bridge. Still separating, the connection is reduced to a filament and becomes very fine.
Radiolaria is an example of Class Actinopoda
Radiolaria live as plankton in the sea, usually in warm waters. When the organisms die the cell disintegrates and leaves the finely sculptured shells figured here. The shells sink slowly into the oceanic ooze where they may become fossilized if conditions are suitable.
This class includes the various species of amoebas. Typically amoebas crawl on the surface of the mud, submerged plants, in the spaces between soil particles, or inside animals as parasites. Locomotion is by means of pseudopodia, which may be long and thin but usually unbranched (filopodia), long, thin, and branched, uniting with one another to form a network (rhizopodia) or shorter, broader, and usually blunt (lobopodia). A generalized cell contains all the usual complement of organelles and, if found in freshwater, a contractile vacuole as well. The shape is highly plastic. There are few or no reference points on the surface or within the cell.
These are naked amoebas with a flagellate stage in their life-history. Perhaps the most studied species is Naegleria gruberi which lives in organically rich soils. It is a small limax-type amoeba (see below) in dry conditions or in high osmotic pressure liquids. When placed in distilled water a rapid transformation occurs: first the posterior (uroid) develops long filaments of cytoplasm which move about; the thickest of these become flagella and as soon as they are fully formed the earlier processes are withdrawn; finally, the two to four flagella migrate to the anterior end and the flagella beat more vigorously until the body breaks its attachment with the substrate.
The flagellate swims with rotatory movement. The reverse process has been shown recently to be very sudden: the flagellate sticks to a surface, the flagellum stops, and after a few minutes is quickly withdrawn and the amoeba crawls away. Dieniamotba is a small (5-18μ) limax-type amoeba sometimes found in the intestine of man. It has been shown recently to have a flagellate phase. Apart from having two nuclei, it is very similar to other entamoebas. Hislomonas is an amoeba with one to four flagella.
The comb and wattles of poultry infected by this parasite, known as ‘blackhead’, turn black. The parasite is flagellated in the lumen of the intestine ‘ but becomes amoeboid upon invading the liver. Masligamoeba is large (150-200μ long) and has both pseudopodia and a flagellum at the same time. The pseudopodia are radial or terminal (posterior). Some species of the genus are free-living, others are parasitic, especially in the gut of the amphibians.
Amoeba is an example of Class Rhizopoda
The living cell has pseudopodia with clear expanding tips, flowing central cytoplasm, and numerous food vacuoles. In living cells the nucleus is not readily seen, but when fixed and stained it becomes conspicuous.
Arcella dentata is an example of Class Rhizopoda
Arcella dentata is a testate amoeba. The shell or test is secreted around the cell. Frequently the amoeba dies and the permanent shell is left behind as in the photograph. Among the genera and species a wide variety of shapes and sculpturing of the shells is found. Testate amoebas are common in freshwater and feed crawling over submerged vegetation.
Typical amoebas (Order Amoebina)
These are amoebas in which a flagellate stage is not known. The size range is great, from very small soil species of 3- 15μ to large, sometimes multinucleate, freshwater species up to 3 mm. in length. Reproduc-tion is believed to be asexual and may be by promitosis (binary fission) or true mitosis.
The habits are both free-living and parasitic. Amoeba rains is probably the most commonly studied organism in schools. It is also known as Chaos &films. But it is doubtful whether the various organisms from all over the world which roughly fit the pictures in textbooks arc the species proteus. This type of amoeba has several large pseudo-podia which contain ectoplasm and endoplasm.
It is usually seen to move in one direction at once and the spent pseudopodia collect as small bumps in the uroid. Also in or near the uroid is the contractile vacuole, while the nucleus may be close but somewhat anterior. In the endoplasm of many species of free-living amoeba are bipyramidal crystals of uncertain function. Few pseudopodia reveal much of taxonomic value because the shape is ever-changing. The genera are distinguished principally by nuclear and cyst structure. Therefore identification is possible only by the specialist. The limax-amoebas are the small amoebas which abound in soil.
They are distinguished from the other free-living forms by their single pseudo-podium which contains relatively more ectoplasm. The smallest are only 3μ across and the majority are under 20μ, though up to 40μ has been reported. They are extremely simple in structure. They feed on bacteria and round up into resistant cysts when food is in short supply or conditions are otherwise unfavorable. The Hartmanellidae are amoebas in this group that show true mitosis.
The parasitic amoebas are typically found in the gut of vertebrates and invertebrates. Frequently there is little morphological distinction between the species found in different hosts. They are mostly 20-40μ in diameter. Amoebic dysentery is caused by Entamoeba hystolytica, which invades the walls of the large intestine and causes loss of blood and excessive secretion of mucus. An almost identical species, Entamoeba colt, is found in the same part of the gut but is non-pathogenic.
The precise condi-tions in which one species gains a numerical advantage over the other are difficult to determine as many apparently healthy humans are ‘cyst-. passers’ and clearly infected carriers. The cysts are most resistant to chemical treatment and even to gentle drying. Amoebic dysentery is therefore easily spread by contamination of food. A related parasite, Entamoeba invadens, causes great damage in snakes and other reptiles. It does not infect man.
Testaceans (Order Testacea)
This order contains amoebas that build a shell or test in which they live. They are distinguished from the foraminifers by having only one test. The type of test determines classification within the order. In some the test is simple, made of secreted substances and not complicated by plates. The genus Arcella belongs here. Its test is pale brown and shaped like the cap of a young mushroom. The organism lives outside the test and extends blunt pseudopodia through a hole where, in the mushroom, the stalk would join the cap. These organisms are 25-100μ and are found on surfaces of decaying plants in freshwater.
Foraminifera Elphidium and Plectofrondicularia are examples of Class Rhizopoda
These photographs illustrate the range of forms found in the group. Only shells are shown. The smallest chamber at the center of spiral types or the thin end of elongate shells is the proloculum.
These are flagella-bearing protozoans which have truly animal methods of nutrition. Many are parasitic and have been extensively studied as they affect the life of man. The number of free-living species is not certain. They are morphologically diverse and are best considered in their orders.
Protomonads (Order Protomonadina)
All members have only one or two flagella, but are otherwise probably unrelated. In the family, Codosigidae are the remarkable collar flagellates. The collar is a contractile cylindrical process from the cytoplasm which covers about one-fifth of the flagellum. Food is apparently directed by the flagellum to a pocket between the cell and its test or shell, traveling down the outside of the collar. The particle is ingested by the movement of the cell against its test.
The genus Bodo (Bodonidae) is common in freshwater. It has two flagella; one leads and the other is bent back round the body so as to trail. It is small, about 14M in diameter, and rather difficult to observe. Stained preparations show that it has a DNA-containing body called a kinetoplast at the base of the flagella.
The kinetoplast is also found in the trypanosomatids, of which one species causes sleeping sickness in man and related diseases in vertebrate animals. Trypanosoma is a genus of variable morph-ology but members usually have one flagellum. Sufficient work has elucidated the relations between apparently dissimilar organisms. Change in shape bears a clear relation to the environment of the parasite and to the evolutionary stage attained by a particular species. For example, a simple insect parasite, which seems to do no harm to its host, like Crithidia gerridis of water bugs (Gerris sp.) is found in crithidial, leptomonad and leishmanial stages.
Trypanosoma brucei which has a life-history in the Tsetse Fly and in cattle is known in all stages except leishmania in the vertebrate. Schizotrypanum cruzi even has a leishmanial stage. Although in the leishmanial form the flagellum is absent, rudimentary organelles remain and from these the new flagellum grows. Slender body and vigorous movement are typical of trypanosomes, which are found in a wide range of vertebrates and invertebrates. As the blood is the easiest organ in which to see moving objects it is not surprising that trypanosomes are considered to be blood parasites. But much of the damage they do is not produced in the blood but is destruction or poisoning of the brain (by T. rhodesiense) or of the heart muscle (by Schizotrypanum cruzi).
The undulating membrane of the textbooks which was supposed to attach the flagellum to the body is not a real membrane but a loose pellicle pulled out by the movement of the flagellum. The names given to the organelles at the base of the flagellum have caused much confusion. The basal granule is the centriole-like structure at the root of the flagellum, while the kinetoplast is the larger, darkly staining body that contains DNA and has recently been shown to be associated with mitochondria. The size of trypanosomatids varies from 2µ in leishmania to several hundred for the trypanosome form of certain marine fish parasites. In the disease called leishmaniasis, the trypanosome form is never seen.
The form found in mammals is the leishmania and the vector, Phlebotonuu sp., develops a leptomonad and so indicates that the causative organism (Leishmania sp.) is related to other trypanosomes.
Metamonads (Order Metamonadina)
These flagellates have more than two flagella. Their other features are variable, as are the number of nuclei. The genus Trichomonas (3-20μ in length) consists of parasites of man and other animals. Typically there are four flagella, three fairly short and projecting forward from their bases, the fourth, although arising with the others, is much longer and is united with the body to form an undulating membrane extending beyond the posterior of the body. In addition there is an axostyle as a stiffening rod running the length of the body. These organisms are found in the gut of many vertebrates and invertebrates where for the most part they do little harm. Trichomonas vaginalis, however, is found in the genital tract of women, commonly in some geographic regions, and to a lesser extent in men. Under certain conditions the numbers become excessive and inflammation results.
Giardia is another genus commonly found in the gut of vertebrates. It is sometimes included in the order Distomatina. The body has a characteristic kite shape and bilateral symmetry is strictly observed in all organelles including the nuclei, of which there are two. There are six flagella. Normally no inconvenience is experienced by the host of these parasites, but occasionally the numbers rise and cause a severe diarrhoea.
Trichonympha and its allies are found in the gut of termites and orthopterans such as cockroaches and woodroaches. They are mutualistic symbionts in that they are essential to the economy of many of their wood-feeding hosts, breaking down cellulose particles that the host’s own enzymes cannot digest. Length ranges between 501.t and 300p. But some species are as small as 5p. The number of flagella is very large.
This genus is not classified among the ciliates because it has only a single nucleus and an amoeboid posterior, which is used to ingest particles of wood. Food vacuoles and their contents are clearly visible in the living organism. It has been shown recently that sexual phenomena occur when the host moults. The onset of meiosis is controlled by the host moulting hormone.
This order even more closely resembles the ciliates than does Trichonympha. Cilia arise all over the body I and are of uniform size with interconnected bases. Opalina used to be classified as protociliates but now the genus is included somewhat tentatively in the zoomasts because the many nuclei are of identical size.
When sexual phenomena occur the cells fuse permanently like gametes and not temporarily for the exchange of nuclei (the latter process known as conjugation). The various species are found in the rectum of frogs and toads. They are generally rather large, ranging from 100-800μ long. Infec-tion is transmitted from one frog by encysted forms produced during the frogs’ breeding season. Recent experiments have shown that sex hormones injected into the host can induce cyst formation though it is not clear whether this has a direct or indirect effect on the parasites.
Trypanosoma brucei ( x 2,320, stained blood smear).
Trypanosomes that cause African sleeping sickness are found in the blood or lymph nodes of patients. The flagellate is about 20it long and thrashes continuously. As can be seen the single flagellum runs almost the entire length of the cell and terminates in a basal granule and the associated kinetoplast. This smear. was made from mouse blood in which the trypanosomes reproduce more vigorously than in man and reach the numbers shown. In mice, they also change from being polymorphic (i.e. having long slender, short stumpy and a variety of intermediate forms) into a monomorphic infection (only one form).
Metamonads (Order Metamonadina)
These flagellates have more than two flagella. Their other features are variable, as are the number of nuclei. The genus Trkhomonas (3-20μ in length) consists of parasites of man and other animals. Typically there are four flagella, three fairly short and projecting forward from their bases, the fourth, although arising with the others, is much longer and is united with the body to form an undulating membrane extending beyond the posterior of the body.
In addition there is an axostyle as a stiffening rod running the length of the body. These organisms are found in the gut of many vertebrates and invertebrates where for the most part they do little harm. Trichomonas vaginalis, however, is found in the genital tract of women, commonly in some geographic regions, and to a lesser extent in men. Under certain conditions the numbers become excessive and inflammation results. Giardia is another genus commonly found in the gut of vertebrates. It is sometimes included in the order Distomatina.
The body has a characteristic kite shape and bilateral symmetry is strictly observed in all organelles including the nuclei, of which there are two.
There are six flagella. Normally no inconvenience is experienced by the host of these parasites, but occasionally the numbers rise and cause a severe diarrhoea. Trichonympha and its allies are found in the gut of termites and orthopterans such as cockroaches and woodroaches. They are mutualistic symbionts in that they are essential to the economy of many of their wood-feeding hosts, breaking down cellulose particles that the host’s own enzymes cannot digest. Length ranges between 50μ and 300μ. But some species are as small as 5μ.
The number of flagella is very large. This genus is not classified among the ciliates because it has only a single nucltus and an amocboid posterior, which is used to ingest particles of wood. Food vacuoles and their contents are clearly visible in the living organism. It has been shown recently that sexual phenomena occur when the host moults. The onset of meiosis is controlled by the host moulting hormone.
This order even more closely resembles the ciliates than does Trichonympha. Cilia arise all over the body and are of uniform size with interconnected bases. Opalina used to be classified as protociliates but now the genus is included somewhat tentatively in the zoomasts because the many nuclei are of identical size. When sexual phenomena occur the cells fuse permanently like gametes and not temporarily for the exchange of nuclei (the latter process known as conjugation). The various species are found in the rectum of frogs and toads.
They are generally rather large, ranging from 100-800μ long. Infec-tion is transmitted from one frog by encysted forms produced during the frogs’ breeding season. Recent experiments have shown that sex hormones injected into the host can induce cyst formation though it is not clear whether this has a direct or indirect effect on the parasites.
The flagellates (Class Mastigophora)
This class contains organisms which are actively motile by means of flagella. They may have a single flagellum or they may have hundreds, so that they bear a strong superficial resemblance to dilates. The nuclei are usually restricted to one per cell but if there is more than one they are all similar and do not display sexual reproduction of the kinds found in the ciliates. They are usually without pseudopodia, and are small (under 100μ). There are both parasitic and free-living members in this class. Reproduction varies: in some species hetero- or iso-gametes are found, in others no sexual processes are yet known.
Phytomasts (Subclass Phytomastigina)
The theoretical distinction between the two subclasses, Phytomastigina and Zoomastigina, is that the former, being plant-like, have chloroplasts and produce their own food with the agency of chlorophyll or related pigments, whereas the latter, true animals, rely on ingesting other micro-organisms. Difficulty arises when a particular organism looks in every way like a phytomast but does not have chloroplasts. Common sense classifies such organisms with those to which they have most resemblance and we presume the colourless forms have lost their chloroplasts. The mastigophorans are most readily thought of as a class made up of some fourteen orders of which eight are of general importance (and dealt with here). Apart from the major classification based on chloroplasts, each order has a characteristic shape and is therefore generally recognisable.
Phytomonads (Order Phytomonadina)
This is the protozoan group clearly the nearest to plants. The chloroplasts contain a bright green chlorophyll and the cell walls, firm and resistant to distortion, are made of cellulose or a close chemical relative. The typical organism is Chlamydomonas, which has two flagella, a pigment spot (stigma), is small (about 20μ) and is found in freshwater. Both iso- and hetero-gametes are known in the different species of this genus. Reproduction by sexual methods is readily induced in the laboratory, and for this reason, the genetical behaviour of the genus is under study. In early spring colonial phytomonads are commonly found in freshwater. These are organisms formed by 4, 8, 16, 32 or more cells, each cell more or less like Chlamydomonas. The arrangement of cells is usually regular, flattened in Gonium, and spherical in Eudorina and Pleodorina.
A larger colonial form consisting of several thousand flagellated cells is Volvox, in which movement is effected by the flagella. Reproduction may be by asexual or sexual method. If by the first, daughter colonies are formed inside the sphere and freed when the mother colony opens; if by a sexual process, male and female gametes occur and the two unite to produce a zygote, or fertilised egg. The zygote is a resistant stage with a thick cell wall, and Volvox usually overwinters in this form.
The niche occupied by most phytomonads is that of small motile plants. Some are most numerous in early spring, when they make use of bright sunshine and the high carbon dioxide concentration of water only a few degrees above the freezing-point. Colonies may be seen actively swimming near the surface of slow streams. One may assume that motility enables them to enjoy an optimum light intensity by day and avoid being frozen at night by descending to lower levels. In contrast to the plant-like behavior of the green phytomonads the genus Polytoma is clearly saprozoic in its nutrition. It has no chloroplasts but is otherwise similar to Chlamydomonas. Since it has no cytostome (cell mouth) the only possible means for food intake would appear to be by diffusion.
Euglenoids (Order Euglenoidina)
The members of this common freshwater order are typically bright green and have only one flagellum, used in locomotion. (In some forms a short second flagellum is seen in stained preparations.) The flagellum arises from within a pit in the anterior end of the cell. Also in this pit, sometimes called a gullet, is a reddish coloured body called a stigma.
A characteristic feature of euglenoids is their food reserve, paramylum, which, although a carbohydrate like starch, does not stain with iodine as do other starches. The cells are otherwise very variable; some secrete flask-shaped loricas (in-animate protective coverings) in which they live, others are naked; some have stiff cell walls, others are very flexible and are shaped by complex changes in subpellicular fibrils; some live on the end of fixed stalks, but most of them are free-swimming.
The genus Euglena has members which range from 30μ to 400μ in length. While most are green, have upwards of fifteen chloroplasts, and are found in freshwater, E. halophila is a marine species and is most tolerant of very high concentrations of salt. E. rubra is reddish and commonly occurs in late summer as the scum on stagnant waters rich in.; organic matter, such as farmyard ponds. Phacus is common in freshwater.
The body is • rigid and flattened in the shape of a conventional% lover’s heart. In some species the body is twisted spirally at the posterior end and produced to a point. Numerous spiral ridges cover the body. Nutritionally Astasia is among the most interesting of the order. Structurally it resembles Euglena but is without chloroplasts. The cells form paramylum when cultured in media devoid of complex substances like sugars or proteins. This property is typical of plants and has been retained despite the loss of chloroplasts. Peranema, like Astasia, is colourless.
It is found in situations rich in organic matter. The flagellum is noteworthy in this genus for it is held out stiff in front of the body. Only the very tip moves and causes locomotion. Related genera have two flagella, one held as described, the other, the shorter one, trails alongside the body.
Cryptomonads (Order Cryptomonadina)
This is a small order whose members usually have two flagella which arise in a pit, are small (15-40p long) and have one or two yellow or brown chloroplasts. The cells, are usually flattened in section. Chilomonas deserves mention because it has an abundance of infusions of plant material although devoid of chloroplasts. It grows so readily that it is frequently used as food for particle feeders, e.g. ciliates.
Chrysomonads (Order Chrysomonadina)
This order is probably large in terms of a number of species and is certainly very large when numbers of individuals are considered. These are the yellowish-brown, very small flagellates ubiquitous in fresh, brackish and seawater. In size they seldom exceed 20p in length. They have one or two flagella, and one to a few chloroplasts.
Owing to their small size, frequently marine habit and the difficulty experienced in making permanent preparations, the study of this order has been largely neglected. Their habits are varied. Colonial forms similar to GonM exist but others form tips of much-branched stalks. Some are solitary. The formation of silica-containing cysts is a feature of the order.
Chromulina is an example of a solitary species. The flagellate stage with one flagellum and one chloroplast is dominant but it also has an amocboid stage. In the latter, the flagellum is entirely absent and locomotion is by means of a single pseudo-podium (a blunt-ended lobopodium). Mallornonas is more elongate than Chromulina and has siliceous spines covering its body. Members of this genus are larger (40-80p) than is general for the order.
Ochromonas has in the past few years become considerable importance as a gauge in the estima-don of the amounts of vitamin 812 present in certain foods. This organism cannot synthesise vitamin and cannot grow without it. Thus extent of growth of a culture on food media affo a measure of the vitamin 1312 content in the food.
Volvox is an example of Class Mastigophora
Volvox ( x 240). The colony shown here is larger than that on page 19, and the larger reproductive cells can be seen to be in different stages of division. The fibrils connecting the smaller cells are also visible.
Volvox ( x 200). A mature colony with fully formed spherical daughter colonies within the parent. The parent subsequently dies or is damaged and on rupture releases the daughter colonies, each now complete with vegetative and reproductive cells.
The organisms have the appearance of a small, round cell with two stubby flagella, one slightly longer than the other.
Dinoflagellates (Order Dinoflagellata)
This comprises a fairly homogeneous assemblage of mostly marine organisms characterised by their sculptured cellulose cell walls. They have two flagella. One lies in a groove on the equatorial plane while the other projects to the posterior of the organism from a groove, the sulcus, confluent with the equatorial groove or girdle. By this arrangement both flagella arise close together though their tips are far apart. The longitudinal flagellum is used for propulsion. The equatorial flagellum rotates the animal and may serve in orientation. Except for Ceratium and Noctiluca the size range is 20-80μ. Although some species have a variety of chloro-phylls and the food reserves are starch and lipids, there appears to be a tendency to holozoic nutrition, that is, taking in food through permanent openings.
Particulate food is ingested in many species and in Ceratium there is evidence that food is also captured by means of a fine pseudopodia] network. Of more doubtful function are the two pusules or vacuoles containing a pink fluid. These are connected to the outside by fine pores and may be used in the intake of fine suspended matter.
Gymnodinium is an example of a ‘naked’ dino-flagellate. No thickened, sculptured plates are carried. Structurally it is probably the simplest of the order. Members of the genus are found in lakes, freshwater ponds and also in the sea. Some are green and holophytic; that is, they make complex organic substances by photosynthesis and from simple substances absorbed through the body surface. Others are holozoic. Occasionally the balance of nature controlling the number of organisms is disturbed and vast numbers are found in some subtropical waters.
For example it occurs on the Florida coast at times in such numbers that fish are poisoned. The species concerned is reddish and the plague is known as the ‘red tide’.
Another red dinoflagellate is Gonyaulax. The genus has armoured plates surrounding the body. The plates are of a characteristic shape and have regularly pitted surfaces. As in the last genus sporadic increases in number occur, giving the sea a reddish tinge and killing fishes and crustaceans. The organism contains a toxic alkaloid.
Ceratium has elaborate armouring. The epicone or covering on the top half of the body has one long process and the hypocone covering the lower half has three. The diameter is 100-700μ. Numerous coloured granules, possibly chloroplasts, are arranged in five groups. Food is captured by pseudopodia] web and a large single pseudopodium which can appear through the sulcus. Food vacuoles are clearly visible in the cytoplasm and contain the remnants of other dinoflagellates, diatoms and phytomasts.
Noctiluca is well known as an organism causing phosphorescence of the sea at night. It is a large and aberrant dinoflagellate up to 2 mm. in diameter and with a permanent tentacle formed for the hypocone. Phosphorescent granules in rows form a close mesh in the cytoplasm. The discharge of light occurs mainly when the water is disturbed.
Parasitic dinoflagellates are known, and these are harmful to fishes and some marine invertebrates. They are all small and usually without flagella and the characteristic structures while in the parasitic phase. It is only during the free-swimming phase that they can be identified as belonging to the order.
The term Protozoa embraces the vast assemblage of living organisms, some free-living, some parasitic, that appear to organise their whole lives as single cells. Estimates of the total number of species are difficult to make because the definition of the term species is particularly unclear in the Protozoa. It is safe to say that over 80,000 species have been described and that there is more than three times this number as yet undescribed. Although a few are visible to the unaided eye, being up to 5 mm. long, the vast majority require a good microscope to reveal them and their structure as they are only from 2μ to 8μ long. Whether they are animal or plant, whether this or that group is truly protozoan, whether they form a subkingdom, superphylum or phylum are all matters that have produced diverse opinions. But the one generally accepted criterion for the inclusion of a particular organism in the phylum is that all functions are confined to a unicellular structure. It is therefore valuable to consider cell construction and function before reviewing the more varied and complex aspects of micro-organisms. Without knowledge of basic structure, it is impossible to begin to make sense of the enormous variety of this versatile and cosmopolitan group. Historically protozoology is relatively recent. Protozoans were first seen in 1674 by Van Leeuwen-hock using a simple microscope. Their small size has meant that knowledge about them has been linked with the slow development of the microscope. Improved techniques and instruments have brought with them a wider understanding of the form, physiology and behaviour of these tiny creatures.
Examples of PROTOZOANS
Prymnesium ( x 2,175) is a marine species that live by photosynthesis. The chloroplasts, extending from the base of the flagella, are pigmented and trap the light energy necessary for building up the living cell. This species sometimes grows huge populations and when it forms the diet of fishes is poisonous.
Ceratium is one of the dinoflagellates which are mostly marine, though freshwater forms occur. The spines are stiff and two flagella are used in locomotion. One lies in the equatorial groove seen in the photograph while the other is in a shallow groove running from the equatorial groove.
The structure on a small scale Certain functions are essential to life in its highest and lowest forms. In man, for example, the essentials are movement in search of food and away from harmful agents; awareness of the environment through the senses and integration of responses; capture and processing of food; respiration through the lungs; controlling the water content and soluble substances of the body by means of the urinary system; and reproduction by means of specialised organs. In man, these six functions demand specialised structures or organ systems in which each organ consists of many minutes, and usually specialised cells. In other words, the cell is the lowest structural denominator of life. In higher forms, some cells are so specialised that they cannot exist or multiply outside the organism of which they are part.
Basic cell plan In its basic form the plan of most cells, including most protozoans, is that shown in the diagram of a hypothetical cell below. It will be seen at once that the traditional definition of a cell as a single nucleus surrounded by cytoplasm is now no more than a convenient simplification. Even Amoeba, one of the simplest known forms of animal life, cannot be reduced to quite such fundamentals. With the recent advent of the electron microscope the cell membrane, the mitochondria, the Golgi apparatus and other structures have been elevated from the area of hypothesis to that of common and certain knowledge. The existence of other structures (ribosomes, nuclear pores and the endoplasmic reticulum) was not even suspected by earlier cytologists using light microscopes.
The cell membrane of Protozoa
The cell membrane is composed of two asymmetrical layers, each one formed by a complex of fatty and protein substances known as lipo-protein. The molecules are composed of fat at one end and protein at the other. In a membrane, they are arranged in such a way that the fatty ends are together and the two protein ends face outwards. The distance between the protein layers is between 8 p and 12 p. This typical structure is known as a unit membrane and forms not only the covering of all animal cells but also their projecting organelles and some internal structures.
Mitochondria of Protozoa
Mitochondria of cells other than those of protozoans are ovoid structures with internal anatomy consisting of cristae or layers of double unit membranes crossing transversely. It is these cristae that characterise mitochondria, and are particularly important, as protozoan mitochondria are seldom ovoid, but generally elongated to various extents. In trypanosomes, mitochondria change in size and function during the life-history. In one phase, the bloodstream form, mitochondria are very reduced and poor in cristae, by which characteristic such forms are distinguished. The function of mitochondria is to release energy in usable form to the cell in the process called cellular respiration.
Chloroplasts in Protozoa
Chloroplasts are structures somewhat resembling mitochondria but which contain colour pigments. They give most plants their characteristic green colour. In some flagellated protozoans chloroplasts are green, but more typically they are yellowish-brown. These organelles serve to absorb energy from sunlight and convert light energy into chemical energy of sugar or starch. This is a process requiring dissolved carbon dioxide and water and is generally known as photosynthesis.
Golgi apparatus in Protozoa
Golgi apparatus and lysosomes are structures about which there is still some controversy. Typically the Golgi body has been shown to consist of flattened saccules arranged like a pile of paper bags. Recent electron micrographs strongly indicate that small bodies called lysosomes develop at the tips of the saccules and are freed into the cytoplasm. They are characterised by their typical content of enzymes, which perform a catalytic function in breaking down many biological substances in acid conditions. Lysosomes seem to function as the start of some digestive processes and also in protecting cells against noxious substances.
Cilia and flagella in Protozoa
Cilia and flagella are organelles of locomotion and have essentially the same basic structure. The unit is the flagellum and is composed of eleven hollow fibres inside a cylinder of 200-300mg in diameter. Two fibres lie together in the centre with nine outer fibres arranged in a ring around them. At the base of a flagellum, where it is inserted into the cell body, the centre fibres disappear, leaving only the ring of nine which may become fused. The fused region is known variously as the basal body or basal granule.
Flagella vary in length from 61.1 to 250 p and are found singly or only a few attached to each cell. Cilia, on the other hand, occur typically in rows, when many hundreds may more or less cover the cell, and arc seldom longer than 30p. in a few ciliates, however, which are described in some detail on p. 28, the cilia are much reduced in number or fused together to form paddle-like membranclles or tufted cirri. Thus it is difficult to generalise and it may well be that the distinction between cilia and flagella is artificial. When cilia occur in large numbers they beat in a co-ordinated way, controlled by additional structures running along, and some between, the rows of cilia immediately below the cell membrane.
Protozoan nuclei are so similar that it is best to describe first the structures of a mammalian nucleus and then draw contrasts with the protozoans. These structures are chromosomes, centrioles and spindle (when the nucleus is dividing), nucleolus, and nuclear membrane with its associated pores.
The chromosomes usually are seen clearly only during cell division, when they appear as sausage-shaped elements. The number in each cell is usually constant within a species but can be as low as four or as high as several hundred. It has been demonstrated that the chromosomes contain deoxyribonucleic acid (DNA), the substance which in all animal cells constitutes the encoded genetic information. Between divisions chromosomes are diffuse in the nucleus and not usually visible.
Centrioles and spindle in Protozoa
The centrioles and spindle, like the chromosomes, become visible only at division. The centrioles are short cylinders and arc frequently regarded as the same type of structure as the basal granule of a flagellum. From them arise numerous hollow fibres to which the chromosomes are attached. In a dividing nucleus there are two diametrically op-posed centrioles (see diagram p. 17). The fibres from each centriole approach each other from the poles.
Nucleolus of Protozoa
The nucleolus is a body within the nucleus. Characteristically it appears between divisions. It contains ribonucleic acid. Between divisions the nucleus can be seen to be made of a mesh of fibres or diffuse granules and one or more distinct bodies called karyosornes or endosomes.
Types of Protozoan locomotion
There are three types of protozoan locomotion.
(1) Protozoan Amoeboid locomotion
Amoeboid locomotion, illustrated here by Amoebas proteus, in which a pseudopodium is pushed out and the nucleus moves into it. Other pseudopodia are produced at the same time and the nucleus subsequently moves into one of these.
(2) Protozoan Flagellate locomotion
Flagellate locomotion presented here in schematised form, is by means of one or more flagella, used oar-fashion, except that the flagellum is flexible on the recovery stroke. This is the method used by the majority of flagellates, but others twist the flagellum so that a vortex is created in the water and the animal is sucked into it. (
Ciliate locomotion is produced by rows of short filaments or cilia that cover the surface of the animal and are interconnected by a network of subpellicular fibrils. The cilia behave rather like flagella, except that the strokes are co-ordinated by the subpellicular fibrils to beat in waves, and this is known as metachronal rhythm.
Hartmanella (x 1,220) is an amoeba with interesting pseudopodia. The tips show fine rhizopodia extending from the broader pseudopodia, and food vacuoles are numerous in the cytoplasm. The nucleus containing the nucleolus is clearly seen in this living specimen.
Nuclear membrane of Protozoa
The nuclear membrane is a typical unit mem-brane surrounding the nucleus and separating it from the surrounding cytoplasm. It was once thought that the nuclear membrane acted as a barrier against the flow of materials across it in either direction. Recent electron micrographs have shown that the nuclear membrane is pierced by numerous pores 8-20mµ in diameter. This fact makes its supposed role ‘as a barrier rather less than certain.
The foregoing is a brief account of the generalised nuclear structure. The variations found in different protozoans inevitably strain this generalisation as they strain most others. Some protozoans appear not to have chromosomes; others appear to divide without either centrioles or spindle; some appear to have no nucleolus; some (typically of the class Ciliata) have two nuclei, and others (the Try-panosomatidac) have a fragment of DNA associated with the flagellum, and so on.
Diversity has two main explanations. First, the protozoans are an ancient group of organisms and have undergone extensive evolutionary diversification; second, some of the diversity may be only apparent either because the descriptions of the many species have been made by different authors using different techniques at different times, or because many of the structures are very close to the limit of resolution of the light microscope and are therefore difficult or impossible to see clearly.
The electron microscope has aided greatly the study of the smaller protozoan components both nuclear and extra-nuclear. In some cases a unity of structure has been demonstrated (unit membrane, the fibrillar structure of cilia and flagella). In other cases a considerable diversity has been shown (e.g. variety of cytoplasmic inclusions).
Symmetry of Protozoa
Applied to other animals the term means the spatial arrangement of parts according to some geometrical design. Thus most animals including man have bilateral symmetry, otherwise called mirror-image or two-fold symmetry. Here, the animal body has mirror-image right and left halves. Adult echinoderms such as starfishes with five-fold radial symmetry (the symmetry of cylinders and wheels), break the more usual pattern.
Among protozoans, the protean nature of the body form of some (amoebas) dictates that they be considered asymmetrical. Others, with a more constant body form, maybe imperfectly bilateral (and this group includes the majority, especially hypotrichous ciliates), radial (choanoflagellates), or even spherical (Heliozoa and Radiolaria).
Energy production in Protozoa
Energy is needed for all life processes. Animals obtain it by enzymatically breaking down complex molecules containing much energy into smaller molecules with less total energy. The difference between the two energy levels is available to the organism if it is capable of using it. The most frequent method of breakdown is cellular respiration, summarised in the overall equation: sugar + oxygen —,carbon + water + energy dioxide
C6H1206 + 6CO2 → 6CO2 +6H2O + energy
Actinopod is an example of protozoa
Actinopod (x 615). Here the food-gathering filopodia are seen fully extended. Most of the length of filopodium is stiffened by a spiral structure recently revealed by electron microscopy. Cytoplasm flows along the filopodia carrying attached food particles.
Respiration in Protozoa
The oxygen required for this aerobic respiration is obtained directly or indirectly from the air by respiration. In those protozoans which require oxygen, it is acquired by aerobic respiration, dissolved in the environmental medium (freshwater, seawater, host’s blood), and enters the cell by simple diffusion through the cell membrane. No special respiratory organs are known in the protozoan orders.
Anaerobic respiration in Protozoa
Anaerobic protozoans (e.g. entodiniomorphs) do not require oxygen to produce energy. These are typically the parasitic species that live in a nutrient-rich but oxygen-poor environment of the host, whose metabolism deals with the waste product, lactic acid. The conversion of sugars to these substances extracts only about one-fifth of the total energy that would be available if the complete process took place. Therefore, to obtain a given amount of energy, anaerobic organisms utilise much more sugar than aerobic species.
Aerobic respiration in Protozoa
Aerobic respiration may be considered as a three-stage process: glycolysis or splitting of simple sugar molecules into two 3-carbon molecules; the tricarboxylic acid cycle, in which the 3-carbon molecules are progressively broken down in the mitochondria to liberate carbon dioxide and some energy; and oxygen transport, the major energy source, in which oxygen is brought into combination ultimately with hydrogen.
The energy so produced is used to make a common cell fuel, adenosine triphosphate or ATP. This molecule gives up energy to whatever synthetic system needs it by the liberation of one phosphate group and becomes adenosine diphosphate or ADP.
Homeostasis and osmoregulation in Protozoa
A cell, whether protozoan or metazoan, has numerous substances within it which are essential for its proper function. Some of these constantly are being broken down and utilised and must therefore be replaced. Also, metabolic activity produces waste products that have to be eliminated if the cell is not to be self-poisoned. The functioning of cells is such that the internal constituents, both of cytoplasm and nucleus, tend to remain constant, except for necessary changes in growth and cell division. The general condition of keeping each cell in a state of constant composition is called homeostasis.
One feature of biological membranes is that they are selectively permeable; water and some small ions and molecules can flow more easily through them than larger molecules, although size is not the only regulating factor. This is the basis for the phenomenon of osmosis. In freshwater habitats it is mostly water molecules that bombard the outside of the cell, while the inside is bombarded by water containing dissolved salts, sugars and proteins.
There is therefore a net gain of water into the cell without any corresponding loss of the larger molecules. If this osmosis of water continued unchecked the cell would distend and finally bunt. Normally this does not happen because energy is spent in the cell membrane keeping the water out, though it is not clear by what mechanism this occurs.
However, in many freshwater species some water gets past this barrier and is removed from the cell through the expenditure of energy by mem-branes which form the pulsating vesicle called the tractile vacuole. This process permits the simultan-eous excretion of waste products. Cell membranes, by using energy, are able to regulate the rate of flow of many kinds of ions and molecules into and from cells.
The mechanisms involved are mostly not understood but are called active transport. The general principle demonstrated is that cells can conserve and in some cases concentrate necessary materials within themselves. The em-phasis lies on the properties of the cell membrane and on the healthy cell being able to keep this organelle in good repair to function properly.
In the Protozoa, this is as varied as the main body forms, amoeboid, flagellate or ciliate.
Descriptions of mechanical processes which often happen at considerable speed cannot replace direct observation or, especially, slow motion cinematographs. Amoeboid movement, a very slow form of progress, has been closely investigated recently and several theories have been proposed. As seen from above, amoebas move by a smooth and continuous process of pushing out a pseudopodium (`false foot’). The nucleus (one of the few reference points) moves slowly into it. In the meantime, newer pseudopodia have been formed and movement is then in the direction of one of them. A newly formed pseudopodium has a clear margin, known as the ectoplasm, and an inner granular endoplasm.
The granular bodies have been shown by the electron microscope to be numerous vacuoles, mitochondria and lysosomes. The earlier sol-gel hypothesis supposed that the ectoplasm at the tip of a growing pseudopodium was thinner and weaker than that surrounding the rest of the organism, especially at the rear. And so the jelly-like ectoplasm (gel) tended to contract, forcing the more fluid endoplasm (sol) forwards and thereby extending the weakest part, the new pseudopodium. A more recent theory has been put forward proposing that the ectoplasm is made up of parallel protein molecules which strengthen it. In the ectoplasm of a forming pseudopodium, a folding and regimentation of these molecules occur near the tip. The result is that endoplasm flows into the tip to occupy the newly formed space.
Still, more recently amoebas have been viewed from the side instead of from above. Seen from this angle, they seem to move on small strut-like projections which hold the main pseudopodia clear of the substratum. This important matter is still under active study. Another type of locomotion related to amoeboid movement is found in the Actinopoda, which have fine stiff filaments projecting from the main cell body. Cytoplasm streams along these filaments, carrying with it bits of matter stuck to the surface. The organic bits serve as food when they reach the main cell body.
Those pseudopodia in contact with the substratum slowly ease the animal along. A third type, flagellate movement, is invariably by means of one or more flagella. The exact method of obtaining the relatively high speeds achieved by such small .creatures (often 250μ per second or about a yard an hour) depends on the species.
The movement of the flagellum is by locally-formed waves which can pass in either direction along the length of the organelle. The waves act on the surrounding water to push or pull the animal forward. Most flagellates use the pulling method of propulsion and the flagellum moves first. Some species move by twisting the forward-projecting flagellum back on itself and then by whirling the tip to create a vortex into which the rest of the animal is sucked. The variations on the method are many. Recent slow-motion tine studies have shown that the flagella of trypanosomatids are subject to waves of contraction passing from the tip to the base. Since flagella can be observed to stop and start, the control for starting a wave presumably originates in the cell and some signal must travel along the flagellum from base to tip to trigger a contraction.
Euglenoid movement is an alternation of the shape of the body of Euglena and other similar flagellates apparently determined by the subpellicular fibrils. It is well developed in Euglena and is used to move the whole body. In others it appears to be less involved in locomotion although its exact function is not known. Yet another method of locomotion is found in the ciliates, which sweep themselves along by using the cilia with which most are more or less covered. Each cilium beats in a characteristic way. In the power stroke the filament is stiff and extended while in the recovery stroke it is bent and offers least resistance to the water. The cilia lie in rows interconnected by a complex network of subpellicular fibrils. The result is that the whole ciliated surface of the animal can beat in a coordinated way.
The term meta-chronal rhythm is used to describe the beating, as the cilia do not all exhibit the power stroke simultaneously but in an orderly sequence. This sends waves of power and recovery strokes alternately down the body. The varied and advanced methods of locomotion found especially in the Spirotrkha are produced partly by the loss of some rows of cilia and partly by fusion of others into cirri and membranellae. Many organisms in this group are able to ‘run’ over the surface of water plants with astonishing speed. The cirri serve as stronger leg-like units than single cilia, and although more widely separated from each other still beat in a coordinated way.
Euglena is an example of protozoa
Euglena(x 850). Binary, fission in this living cell is taking place by longitudinal splitting. In this way not only is the genetic material contained in the nucleus halved between each cell, but also the cytoplasmic particles, chloroplasts, food reserves, and mitochondria are equally divided.
Euglena ( x 800). The same cells have nearly completed division. When complete flagella are developed the separated cells swim away.
Vorticella is an example of protozoa
Vorticella has its main body on the end of a contractile stalk shown here shortened by forming a spiral. Feeding is achieved by ingesting particles wafted in the current of water created by the crown of cilia that can be seen around the top of the body.
Individual cells of this euglenoid species form stalks of mucilage. While they are in this pseudo-colonial phase there are no large flagella, but the cells can separate, grow a flagellum and swim around like others. flagellates. Euglena ( x 850). Binary. fission in this living cell is taking place by longitudinal splitting. In this way not only is the genetic material contained in the nucleus halved between each cell, but also the cytoplasmic particles, chloroplasts, food reserves, and mitochondria are equally divided.
Euglena ( x 800). The same cells have nearly completed division. When complete flagella are developed the separated cells swim away.
Nutrition of Protozoans
In the Protozoa this is so varied that it serves to emphasise their claim as founders of the animal kingdom. Some, like Euglena, are green and are very close to plants, some like Didinium ingest large particles or the whole bodies of other protozoans, some ‘drink’ in nutrients by forming small pockets in the cell membrane, some filter off particles in the surrounding water by setting up wide-ranging currents, and there are some in which the feeding mechanisms are still obscure. The commonest methods of obtaining food are undoubtedly filter feeding, phagocytosis—both phagotrophic feeding methods—and saprozoic feeding. Phagotrophy is the general term applied to feeding on particulate matter. If the particles are in suspension then the method usually employed is filter feeding.
As practiced by Vortieella, a sessile ciliate, a current of water is produced by the beating of the cilia around the crown of the body, so causing a vortex that draws particles into the mouth. Particles may also be taken when they are on the surface of water plants or in among other materials such as soil or intestinal contents.
The process is a modification of amoeboid movement and consists in forming a hollow conical pseudopod with the particle in the depression. The pseudopod rejoins with itself on the other side of the particle, which is now enclosed within the protozoan. This process is known as phagocytosis.
Saprozoic feeding was earlier thought to be diffusion of soluble food through the cell membrane. Two things have changed this concept considerably. First, the distinction between a solution and a suspension is no longer clear cut. For example, protein molecules which appear to form a stable solution under normal conditions can be thrown down to the bottom of the vessel by ultra-centrifugation. The distinction, then, is one of convenience: small particles (e.g. proteins) cannot be seen when in solution; larger particles can be seen and the mixture is then called a suspension.
The second point is that more careful observation of cells revealed pinocytosis (cell drinking), a process in which the cell membrane, instead of absorbing material in solution, forms very deep cones, smaller and more tube-like than in phago-cytosis, into which the solution is drawn. The tube closes behind it and so cuts off pinocytosis vacuole. Pinocytosis may be considered, rather crudely, as a modification of phagocytosis or vice versa. The mechanisms are similar but are far removed from
Mitosis in Phylum Protozoa
Mitosis is the usual process during which a cell divides into two. At first the nucleus is resting (A), then the chromosomes appear as threads (B) which shorten and coil into spirals. At this point the centrioles or poles begin to move apart, and a spindle forms between them. The chromosomes lie across the spindle (C), attached to it by their spindle attachments. By now the nuclear membrane has dissolved (D), and the duplicates of each chromosome separate and move towards the poles (E) and the spindle itself elongates, pushing the groups of chromosomes further apart. The chromosomes uncoil (F), elongate and disappear, and a new nuclear membrane forms to give two distinct new cells.
This class is characterized by non-motile spores, each containing two or more polar filaments, small sacs with curled spring-like structures within. They are all parasitic and small, with a host range that includes both invertebrates and vertebrates. The infections caused are mostly mild or occur in animals which are not important in human economy. An exception is filosema (order Microsporidia) of silk-worms and honey bees, where a debilitating and often fatal disease results from infection.