INVERTEBRATE ZOOLOGY - H. Y. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, or BILATERIA

SUBSECTION SPIRALIA

PHYLUM PLATHELMINTHES

SUBPHYLUM NEODERMATA

SUPERCLASS ACERCOMERA

CLASS ASPIDOGASTREA

Aspidogastreans are a small (around 40 species) and poorly studied group of parasitic worms, previously classified under the Class Trematoda. They parasitize Mollusks and aquatic vertebrates (fish and turtles). Their body length does not exceed 2-5 mm.

Morphology. A distinctive feature of aspidogastreans is the presence of a large adhesive disc on the ventral side of the body, divided into numerous alveoli (up to 70 or more), each surrounded by a muscular rim and capable of functioning as a small independent sucker (Fig. 132, a). It has been established that the adhesive disc performs other Functions as well: it secretes mucus that facilitates animal locomotion, as well as Enzymes involved in Digestion and nutrient transport.

Fig. 132. Aspidogaster conchicola (from Strelkov): a - adult specimen; b - larva: 1 — adhesive disc;

2 - suckers; 3 - Pharynx; 4 - intestine; 5 - protonephridia

The anterior end of the body features the Mouth opening, surrounded by a muscular rim. It leads into an unbranched, sac-like intestine; the excretory system is of the protonephridial type, and The Nervous system is orthogon-type. The Structure of the hermaphroditic Reproductive System resembles that of trematodes.

Life Cycle. In species parasitizing mollusks (primarily bivalves, more rarely gastropods), The life cycle is simple. The mollusk serves as the sole host for these species. Larvae hatch from eggs released into the Water. They lack cilia and move via muscular contractions. At the posterior end, the larva possesses a large sucker (Fig. 132, b). Infection of the host occurs passively: larvae are carried into the mantle cavity of the mollusk with the water current and migrate through the host's body until they reach their final localization site (mantle cavity, Pericardium, Kidneys, or other Internal Organs). During metamorphosis, the posterior sucker transforms into the adhesive disc.

Much less frequently, in species that parasitize vertebrates as adults (in the intestine of fish or The Stomach of marine turtles), the life cycle involves a change of hosts. Ciliated larvae hatch from eggs and swim in the water until they enter the mantle cavity of the intermediate host—a bivalve mollusk. Subsequently, if the mollusk is consumed by a fish or turtle (definitive hosts), the parasites inhabit their intestine. In many species, sexual maturation can also occur within the intermediate host; thus, vertebrates in this case play The Role of facultative (optional) hosts. Representative Examples of the class include Aspidogaster conchicola, a parasite of bivalves, and Aspidogaster limacoides, a parasite of freshwater Fishes.