INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM PROTOZOA, OR UNICELLULAR ORGANISMS (PROTOZOA)

PHYLUM CILIATED PROTOZOA, OR CILIATES (CILIOPHORA)

CLASS POLYHYMENOPHOREA

Ciliates of this Class are characterized by the presence of a right-hand spirally coiled zone consisting of numerous adoral membranelles. In addition to the adoral spiral, there are cilia that either evenly cover the entire body or form various cirri, predominantly on the ventral side. This class comprises five orders.

Order Heterotrichida

Representatives of this order are free-living planktonic and benthic sessile forms. The entire body of these ciliates is covered with short cilia, whereas the adoral membranelles consist of longer cilia. In terms of their Structure and way of life, these two groups of cilia do not differ from one another.

Ciliates of the genus Stentor are among the largest (up to 2–3 mm) freshwater ciliates. Their body is funnel-shaped with a stalk at the posterior end, which the animals use to attach themselves to the substrate. When disturbed, Stentor contracts sharply, taking on an almost spherical shape. Like other members of the order, they feed on Bacteria, and the Cytoplasm contains numerous vacuoles: in addition to the contractile and digestive vacuoles, There are also vacuoles filled with Cell sap. The body of S. polymorphus often has a green color due to the presence of symbiotic unicellular green Algae living within it. In S. coeruleus, the body is colored blue owing to a pigment contained in granules. Some heterotrichs (Blepharisma) possess red pigments. Freespanning freshwater species also include the large (up to 2 mm) oval Ciliate Bursaria truncatella and worm-like species of the genus Spirostomum (see Fig. 46).

Sessile species (Eufolliculina and others) dwell in peculiar cases made of organic matter secreted by them. The peristome is elongated into a pair of wing-like outgrowths surrounded by membranelles.

Order Oligotrichida

These are small ciliates that inhabit mainly the Water Column. Among them, there are benthic forms and endobionts of the intestine of sea urchins. Their adoral membranelles are very strongly developed and perform not only a trophic but also a locomotory function; the body cilia are strongly reduced, often gathered into tufts, as in Halteria (Fig. 61).

Fig. 61. Order Oligotrichida: diagram of The structure of Halteria:

1 — long membranelles used for swimming; 2 — short membranelles that drive food toward the oral opening

Order Tintinnida

This order includes over 300 species of marine planktonic forms; their ciliature resembles that of oligotrichs. The body of tintinnids is housed inside a lightweight yet durable case made of organic matter, into which inorganic particles may be embedded (e.g., Tintinnopsis). Several species of this order inhabit Lake Baikal.

Order Hyptrichida

This order unites dorsoventrally flattened bottom-dwelling ciliates. On the ventral side of the body, there are massive structures formed by fused cilia—cirri—by means of which the animal crawls along the substrate. There is also an adoral zone of membranelles and small dorsal cilia. The order includes a relatively large (0.3 mm) freshwater ciliate Stylonichia mytilus, which has a wide feeding range—from Protozoans (flagellates, small ciliates) to bacteria (see Fig. 46).

Order Odontostomatida

This order comprises highly specialized ciliates that inhabit mainly sapropel (silt composed of diatom remains). Their body is laterally compressed, and the cortex possesses rigid supporting elements that form a shell bearing tooth-like outgrowths.

The cilia of odontostomatids are reduced; there are only a few membranelles near the Mouth and short, solitary cilia scattered across the body. A representative of this order is Discomorpha (Fig. 62).

Ciliates are among the most complexly organized unicellular organisms. Presumably, they evolved from flagellates As a result of three major aromorphoses: nuclear dualism, polymerization of flagellates and The Emergence of ciliary locomotion, as well as the complication of cortical structure. Nuclear dualism, which ensures the Separation of vegetative and generative Functions, led to the appearance of a specialized macronucleus with a high degree of ploidy in a portion of its genome, which in turn accelerated Protein Synthesis AND, accordingly, growth with a general intensification of functions. Ciliary locomotion proved advantageous for swimming (planktonic forms) and for moving along the substrate using closely spaced cilia—cirri (benthic forms). Membranelles, formed by rows of cilia, serve as an efficient apparatus for filtering food particles from the water and transporting them to the cytostome. The supporting structures of the cortex act as a peculiar Cytoskeleton that gives the body a constant shape and flexibility. Ciliates have mastered all types of water bodies and most biotopes within them (the water column, silt, sandy bottoms, etc.). In addition to filter feeders, there are known predators that engulf large prey (Didinium) or suck it out (suctorian ciliates).

Fig. 62. Order Odontostomatida: Discomorpha with spike-like outgrowths (1)

Many ciliates from various groups have transitioned to exo- or endobiosis. They utilize the host's body as a habitat, yet they feed primarily on bacteria rather than on the host's nutrients. Based on this way of life, Various Forms of parasitism arise—facultative tissue parasitism of obligate endobionts (Balantidium), obligate parasitism in the integuments of fish (Ichthyophthirius), or cavity parasitism predominantly in the intestine of worms (astomatids). It should be noted that among ciliates, the number of true parasite species is insignificant; most forms considered parasitic are actually exo- or endobionts.