INVERTEBRATE ZOOLOGY - H. Y. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOANS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA), OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIANS (SPIRALIA)

PHYLUM FLATWORMS (PLATHELMINTHES)

SUBPHYLUM NEODERMATA

SUPERCLASS ACERCOMERA

CLASS ASPIDOGASTREA

Aspidogastrians are a small (about 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 size does not exceed 2-5 mm.

Morphology. A distinctive feature of aspidogastrians is the presence of a large ventral attachment disc 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). Research has shown that the attachment disc also performs other Functions: it secretes mucus, which facilitates locomotion, and Enzymes involved in Digestion and nutrient transport.

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

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

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

Life cycle. In species that parasitize mollusks (mainly bivalves, less commonly gastropods), The life cycle is simple, with the mollusk serving as the sole host. Unciliated larvae hatch from eggs released into the Water and move by muscular contractions. The posterior end of the larva bears a large sucker (Fig. 132, b). Infection of the host occurs passively: larvae are carried into the mantle cavity of the mollusk by water currents and migrate through the host's body until reaching their final site of localization (the mantle cavity, Pericardium, Kidneys, or other Internal Organs). During metamorphosis, the posterior sucker transforms into the attachment disc.

Significantly rarer is the life cycle involving alternation of hosts in species that parasitize vertebrates as adults (in the intestine of fish or The Stomach of marine turtles). Ciliated larvae hatch from eggs and swim in the water until they enter the mantle cavity of an intermediate host—a bivalve mollusk. When the mollusk is consumed by a fish or turtle (definitive hosts), the parasites settle in their intestine. In many species, sexual maturation can also occur within the intermediate host; thus, vertebrates act in this case as facultative (optional) hosts. Representative species of the class include Aspidogaster conchicola, a parasite of bivalves, and Aspidogaster limacoides, a parasite of freshwater fish.