Invertebrate Zoology: A Textbook - T. A. Dauda 2014

Multicellular Animals
Molluscs, or Soft-bodied Animals

General characteristics

Molluscs comprise over 128,000 species. They represent the second-largest animal phylum after Arthropods, originating from segmented worms. Molluscs inhabit primarily aquatic environments, populating seas, oceans, and partly freshwater bodies. Only a few species have adapted to terrestrial life.

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

The body of a mollusc typically consists of three regions: the HEAD, more or less distinctly separated from the trunk; the visceral mass, containing most of the Internal Organs; and the FOOT—a muscular part of the body, most commonly shaped like a broad muscular sole or a pointed keel (Fig. 38). Using the foot, the mollusc moves slowly or attaches itself to a surface.

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Fig. 38 External Anatomy of the swan mussel (Anodonta cygnea):

1 — foot; 2 — inhalfferent (branchial) siphon; 3 — exhalant (anal) siphon; 4 — umbo of the shell (the conchiolin layer is eroded, revealing the inner nacreous layer); 5 — ligament.

A shell is typically present on the DORSAL SIDE OF the body, usually unitary, less commonly bivalve or plate-like. The shell generally consists of 3 layers: a thin outer organic layer; a middle, thickest prismatic or porcelain-like layer composed of calcium carbonate columns; and an inner nacreous (mother-of-pearl) layer formed by overlapping calcium carbonate lamellae. The layered Structure OF THE inner layer creates light Interference, giving mother-of-pearl its iridescent rainbow colors.

The growth of the mollusc is accompanied by the growth of the shell, all layers of which are secreted by different Regions of the mantle. The mantle is a dorsal Skin fold that expands significantly and usually envelops all or part of the mollusc's body.

A space known as the mantle cavity lies between the mantle and the body. It houses the gills, chemoreceptors, anus, and other structures, collectively forming the mantle organ complex. The mantle cavity communicates with the external environment via siphons or, in terrestrial molluscs, a breathing pore (pneumostome).

Internally, molluscs are characterized by spaces between organs filled with loose tissue, or parenchyma (hence the phylum name, soft-bodied). Nevertheless, molluscs are coelomate animals: their coelom persists as the pericardial cavity and the cavities of the Gonads. Specialized musculature moves the foot and shell, while a cutaneous-muscular body wall is absent.

The Digestive System of molluscs consists of 3 sections: the foregut, midgut, and hindgut. Most molluscs feature a tough muscular Tongue with tiny Teeth—the radula—located in the Pharynx. A Liver is present, with its ducts opening into The Stomach, which, together with the intestine, forms part of the midgut.

The respiratory organs of most molluscs are gills situated within the mantle cavity. Terrestrial species breathe using Lungs, which represent a modified mantle cavity.

The Circulatory system OF molluscs is open, meaning Blood circulates not only through Blood Vessels but also through a system of spaces located between Tissues and organs within the parenchyma. However, these animals possess a complex pulsating organ—a multi-chambered Heart consisting of 2 atria and a ventricle. It is located within the pericardial sac.

The excretory organs of molluscs are represented by specialized Kidneys, with one end opening via a ciliated funnel into the pericardial sac and the other into the mantle cavity.

The Nervous system in most molluscs is of a scattered ganglionic (or decentralized) type: several pairs of ganglia are located in different PARTS OF THE body and interconnected by long longitudinal connectives.

Molluscs reproduce exclusively sexually. Most are dioecious, though hermaphrodites are also common. Development typically involves multiple larval stages, whereas many freshwater and terrestrial forms exhibit direct development.

THE PHYLUM MOLLUSCA comprises 7 classes, but representatives of only 3 are numerous and well-known: Lamellibranchia or Bivalvia (bivalves), Gastropoda (gastropods), and Cephalopoda (cephalopods).