INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION SPIRALIA

PHYLUM MOLLUSCA

CLASS BIVALVIA

SUPERORDER PROTOBRANCHIA

Exclusively marine forms distributed worldwide across the World Ocean at virtually all depths, from the littoral zone to deep-sea trenches

(deeper than 8 km), and at great depths they constitute one of the dominant mollusk groups. They are absent in enclosed seas with low salinity, such as the Azov, Aral, and Caspian Seas.

This superorder unites Mollusks with primitive structural features. The shell hinge consists of uniform pointed Teeth positioned perpendicularly to the hinge margin; in some species, the teeth are reduced. The gills take the form of bipectinate ctenidia (Fig. 219). The Stomach has a relatively simple Structure. The FOOT features a broad, flat sole. When retracted into the shell, the sole folds longitudinally along the midline. The labial palps are well developed, formed by a pair of leaflets and a long tentacle-like appendage used by the mollusk to gather detritus. Protobranchs lack a byssus and do not attach to a substrate. By feeding method, they are detritivores.

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Fig. 219. Anatomy of the slender nut-clam (Leionucula tenuis) (from Dogel): a - right valve from the dorsal side; b - general view of the body (right valve and mantle fold removed): 1 - taxodont hinge teeth; 2 - pedal Muscles; 3 - projections of the mantle margin between the teeth; 4 - visceral mass;

5 - anterior adductor Muscle; 6 - region of the right mantle fold; 7 - Mouth; 8 - labial palp; 9 - foot; 10 - foot sole; 11 - left mantle fold; 12 - appendage of the labial palp; 13 - right ctenidium; 14 - posterior adductor muscle

Protobranchs include small bivalves, such as nut-clams (Nucula); *Yoldia*, *Portlandia*, and *Nuculana* are also widespread in northern seas. The latter form dense populations and thus play a significant role in benthic biocenoses. Species of the order Solemyida live within the mud strata without direct contact with open Water—a very rare occurrence among mollusks.