Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Mollusks

General characteristics

Molluscs comprise over 128,000 species. They represent the second largest animal phylum after Arthropods, tracing their evolutionary origin back to segmented worms. Molluscs predominantly inhabit aquatic environments, populating seas, oceans, and partially fresh waters. Only a small number of species have adapted to terrestrial life.

Molluscs are bilaterally symmetrical animals, although some exhibit Asymmetry to varying degrees.

The body of a mollusc typically consists of three regions: the HEAD, which is more or less distinctly separated from the trunk; the visceral mass, containing the majority of Internal Organs; and the FOOT, a muscular body part most commonly shaped like a broad muscular sole or a pointed keel. The mollusc uses its foot to slowly crawl or attach itself to various surfaces.

The Skin on the dorsal side forms a more or less expanded fold—the mantle—which covers the body. The shell is a product of the outer mantle epithelium. The mantle cavity formed between the mantle and the body houses the respiratory organs (gills), the openings of the excretory and reproductive systems, and the anus. The secondary body cavity (coelom) is significantly reduced; in adults, only two remnants persist: the pericardial cavity and the gonadal cavity. The spaces between the organs are filled with parenchyma. The Circulatory system is open. Blood Circulation is driven by a Heart consisting of an atrium and a ventricle. The excretory organs of molluscs are specialized Kidneys, with one end opening via a ciliated funnel into the pericardial cavity and the other into the mantle cavity, communicating with the external environment. The Nervous system in most molluscs is of a decentralized ganglionated type, featuring several pairs of ganglia located in various PARTS OF THE body and connected by long longitudinal connectives.

Molluscs reproduce exclusively sexually. The majority are dioecious, though Hermaphroditism is also widespread. Development typically involves several larval stages, whereas A number of freshwater and terrestrial forms undergo direct development.

THE PHYLUM MOLLUSCA encompasses 7 classes, but only three of them include numerous and well-known representatives:

1) Bivalvia (Lamellibranchia). Marine and freshwater molluscs possessing a bivalve shell. Representatives include the swan mussel (Anodonta), river mussel (Unio), oyster (Ostrea), mussel (Mytilus), etc.;

2) Gastropoda. The largest and most diverse group of molluscs, represented by marine, freshwater, and terrestrial forms. They feature a well-developed head with tentacles and a spirally coiled shell, predominantly conical in shape. Representatives include pond snails (Lymnaea stagnalis, L. truncatula), ramshorn snails (Planorbis), the Roman snail (Helix pomatia), etc.;

3) Cephalopoda. Large marine soft-bodied molluscs with the most complex Organization. Representatives include the octopus (Octopus), cuttlefish (Sepia officinalis), and squid (Loligo edulis).