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.