INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOANS (PROTOZOA)

FLAGELLATES

PHYLUM POLYMASTIGOTA

CLASS PARABASALEA

Parabasalids are commensals and parasites of animals and humans. More than 300 species are known.

Structure. The Specificity of the Class is determined by a special parabasal complex of Organelles, which includes the Golgi apparatus and parabasal filaments associated with the kinetosomes of the flagella. The flagella (ranging from one to tens of thousands in different species) originate from the anterior end of the body. Parabasalids possess a unique organelle called the axostyle, which consists of numerous microtubules and serves as a skeletal element of The Cell, anchoring The Nucleus within it. There may be a single axostyle or multiple ones. The single nucleus is located in the anterior part of the cell and is connected to the axostyle. The complex karyomastigont of parabasalids includes the kinetosomes, the flagellar ROOT system, the axostyle, and the nucleus. Parabasalids reproduce asexually.

Parabasalids live exclusively inside other organisms, and some of them are agents of severe diseases. The class includes two orders.

Order Trichomonadida unites commensals and parasites of the intestine and other Internal Organs of vertebrates and humans. Four to five flagella are located at the anterior end of the cell, of which one is directed backward and forms an undulating membrane with the pellicle, while the remaining flagella are free (Fig. 14). A special organelle, the costa, consisting of microtubules, is situated beneath The cell membrane and serves as a support for the undulating membrane, whereas a tubular supporting structure, the axostyle, runs along the axis of the cell internally.

Fig. 14. Trichomonas termopsidis (from Hausmann):

1 - undulating membrane; 2 - flagellum; 3 - nucleus; 4 - costa; 5 - parabasal filament; 6 - axostyle

Humans can host Trichomonas vaginalis (a parasite of the Urogenital System that causes inflammatory processes) and Pentatrichomonas hominis (an intestinal parasite). Tritrichomonas foetus parasitizes the Reproductive System of cattle, causing Infertility or Spontaneous Abortion of the embryo.

Order Hypermastigida includes mutualistic symbionts inhabiting the intestines of wood-feeding insects (termites and wood-eating cockroaches). They possess numerous flagella, which are either gathered at the anterior end of the cell or cover its entire surface (Fig. 15).

The single nucleus is located closer to the anterior end of the cell, and there is one or several axostyles. Hypermastigids ingest wood particles using pseudopodia. The mass of hypermastigids can constitute up to a third of the host insect's body mass.

Hypermastigids synthesize the enzyme cellulase, which breaks down Cellulose, whereas the insects themselves either do not produce this enzyme or produce it in insufficient quantities. The flagellates consume only a portion of the cellulose breakdown products, while the rest goes to nourish the host. Termite larvae feed on the excrement of adult insects and thus become infected with the symbionts. One of the representatives of this order is Teratonympha mirabilis (Fig. 15, a).

Fig. 15. Hypermastigids from the termite gut (from Poljansky): a - Teratonympha mirabilis; b - Rhynchonympha tarda:

1 - parabasal bodies; 2 - flagella; 3 - outer cap; 4 - nucleus;

5 - flagellar ribbons; 6 - axostyles; 7 - digestive vacuoles containing wood particles