Zoomasts (Subclass Zoomastigina) Examples and characteristics

Zoomasts (Subclass Zoomastigina)

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.

Order Opalina

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).

Trypanosoma brucei Zoomasts (Subclass Zoomastigina)
Trypanosoma brucei Zoomasts (Subclass Zoomastigina)

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.

Order Opaline

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.

 

Leave a Comment