INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - G.Y. Shcherbak - 1995

SUBKINGDOM PROTOZOA

PHYLUM CILIOPHORA

CLASS KINETOFRAGMINOPHOREA

This Class comprises the most primitive Ciliates whose bodies are uniformly covered with cilia. Their oral area lacks membranelles. In many species, the cilia located near the cytostome are better developed than those on other PARTS OF THE body. The Mouth in representatives of this class is terminal or shifted to the lateral surface of the body. Many of them feature a well-developed skeletal rod apparatus around the mouth. This class includes six orders.

Order Gymnostomatida

These are predominantly predatory ciliates whose mouth is located on The Cell surface, typically at the anterior end of the body; a peristome is absent (Fig. 52).

Fig. 52. Order Gymnostomatida: Didinium nasutum preying on Paramecium

Didinium nasutum is a small (110–150 µm) freshwater Ciliate with a proboscis-like anterior end terminating in a mouth. Didinium feeds primarily on paramecia, which are larger than itself. It first bores into the victim's body using its proboscis, and then, by widening its mouth, swallows the paramecium whole, Swelling significantly. The prey is digested within 2 hours.

Over the course of a day, a didinium consumes 12 paramecia.

Worm-like species of the genus Trachelocerca inhabit the capillary spaces between sand grains on the sea floor. They feed on other interstitial (psammophilic) ciliates. Interestingly, ciliates from various taxonomic groups inhabiting sand possess a worm-like body shape—an adaptation for locomotion within very narrow (capillary) spaces between sand grains.

Order Chonotrichida

This order includes sessile ciliates living on crustaceans, for example, on the gill covers of amphipods. They are not parasites; they feed on small particles filtered from the Water and use crustaceans as a means of transport and a refuge from enemies.

In adult individuals, cilia are found only on a specialized oral Structure called the collar. They drive food into the mouth. Chonotrichids reproduce by budding, producing ciliated, dispersing swarmers. Spirochona gemmipara lives on the gill plates of amphipods (Fig. 53).

Fig. 53. Order Chonotrichida: Spirochona gemmipara on the gill plates of an amphipod

Order Apostomatida

Members of this order are endobionts of crustaceans, Annelids, and Cnidarians. They live predominantly in the intestines of their hosts, feeding on their food. The cytostome and Pharynx of apostomatids are strongly reduced (Fig. 54). The cysts of the ciliate Spirophrya subparasitica attach by a stalk to the body surface of various small planktonic crustaceans. When a marine hydroid polyp consumes a crustacean, small ciliates emerge from the cysts and feed intensively on the crustacean being digested by the polyp (trophont stage). Within an hour, they increase in size three to four times and, after some time, are expelled by the polyp along with the crustacean remains into the environment. The trophont swims to the Base of the polyp, where it attaches to the pedal disc and encysts (gamont stage). The gamont does not feed; inside the cyst, it reproduces by division to form small swarmers that emerge from the cyst, swim, and encyst on the body surface of planktonic crustaceans.

Fig. 54. Order Apostomatida: Foettingeria

Order Trichostomatida

This order includes Bacteria-feeding ciliates inhabiting soil or water bodies, as well as parasitic forms. Their cytostome lies at the bottom of a special depression into which food is driven by specialized membranelles. Trichostomatids can easily encyst. Balantidium coli inhabits the Large Intestine of humans, feeding on bacteria, starch grains, and other organic particles (Fig. 55).

Fig. 55. Order Trichostomatida: a — Colpoda; b — Balantidium coli

Sometimes Balantidium penetrates the intestinal wall, where it feeds on red Blood Cells and other host cells, causing bloody diarrhea. Its invasive stage is the cyst, which is released into the external environment. Balantidium typically inhabits the intestines of pigs without causing any disease in the host. Apparently, the primary host of this species is the pig, whereas human infection occurs accidentally.

Order Entodiniomorphida

Representatives of this order live as endobionts in the rumen of ruminant artiodactyls (Entodinium, Epidinium, Ophryoscolex, etc.), the intestines of horses (Spirodinium), and the large and cecal intestines of anthropoid apes (Troglodytella). They feed primarily on bacteria. By being digested, for instance in the abomasum of artiodactyls, they serve as an additional source of protein for the host. In addition to regular cilia, these ciliates often develop cirri or complex cortical Organelles that form spike-like outgrowths (Fig. 56). Entodiniomorphs do not exert any noticeable effect on their hosts: animals deprived of these ciliates show no changes in Digestion or other physiological processes. Recently, it has been discovered that under extreme conditions, these ciliates serve as a source of water for their hosts.

Fig. 56. Order Entodiniomorpha — Entodinium simplex (a); Anoplodinium denticulatum (б); Polyplastron multivesiculatum (в); Ophryoscolex caudatus (г); Epidinium ecaudatum (д); Anoplodinium bubalidis engulfing a wood particle (е): 1 — micronucleus; 2 — macronucleus; 3 — oral membranelle zone; 4 — contractile vacuole;

5 — cilia; 6 — skeletal plates

Order Suctorida

Ciliates of this order are sessile predators that lack cilia and hunt using tentacles. About 100 species are known, grouped into the order Suctorida. These are predominantly sessile solitary and colonial species. Some representatives inhabit the intestines of horses as endobionts (Alantosoma). Suctorians possess gripping and sucking tentacles (Fig. 57). The gripping tentacles kill prey using toxocysts, whereupon the Cytoplasm of the prey and the predator fuses. The sucking tentacles are expanded at the tip and, in addition to toxocysts, contain an internal tube lined with microtubules: the prey's cytoplasm is drawn into the tube and subsequently phagocytosed. Suctorians reproduce by exogenous or endogenous budding, resulting in a ciliated swarming larva that swims for some time before attaching to a substrate, losing its cilia, and transforming into an adult suctorian. Species of the genus Sphaerophrya are widespread in freshwater bodies.

Fig. 57. Order Suctorida:

a — Dendrosoma radians (colonial form); б — Sphaerophrya magna, feeding on five ciliates simultaneously; в — Dendrocometes paradoxus; г — Allantosoma intestinalis from the horse intestine; д — diagram of budding in Acineta;