Although highly diverse, all members of the phylum share certain general features. Most have a well-developed head, which may bear sensory tentacles; in some, like the clam, the head is very reduced.The Body Wall
All mollusks possess a flexible body wall, which surrounds a body cavity containing the internal organs. The wall, which varies greatly in shape in different species, is usually folded to form a structure called the mantle, which is attached at the top of the body and surrounds it like a tent; the shell is formed on the outside of the mantle. On the underside of the body the wall is usually stretched out to form a thickened mass called the foot. The wall is covered by an outer epidermis and an underlying dermis. The epidermis usually contains gland cells that secrete mucus, which in mollusks has a variety of important uses, such as locomotion, food entrapment, and prevention of water loss. Muscle tissue is found in the body wall, and is particularly plentiful in the foot, which is used for locomotion in most mollusks (although some swim and some are sedentary), and in the mantle in species with reduced shells.
The shell is formed by secretions of glandular cells in the mantle. Except in the chitons, the shells of all mollusks are basically similar, differing only in certain mineralogical details. The shell is composed of an outer, prismatic layer containing densely packed cells of calcareous material secreted by the edge of the mantle; and an inner, nacreous layer of thin, laminated plates of calcareous material laid down by the entire mantle surface. When very thin, the nacreous lining of the shell is pearly and iridescent. Layers of this material may form around a grain of sand or other irritant that lodges between the mantle and the shell; this process eventually forms a pearl. Pearl oysters of the genus Pinctada are the most commercially important pearl formers.
The digestive tract of the Mollusca is complex. The foregut region consists of an esophagus and a mouth cavity, which contains a toothed belt called the radula, found in almost all mollusks and peculiar to the phylum. The radula is usually used for scraping food, such as algae, from surfaces. The number and form of radula teeth are highly variable; some species have a single radula tooth while others may have several hundred thousand. In some the teeth are hollow and poison-containing and are used as weapons; other radula modifications exist. The stomachs of mollusks are generally complex, and these, too, differ with the species and according to the feeding habits of the animal.
Respiration is through gills called ctenidia (sing. ctenidium), located in the mantle cavity (the space between the mantle and the body wall proper) and varies with the species and with the type of habitat. For example, intertidal marine mollusks are exposed to air and water alternately and must be able to respire in both conditions; terrestrial species have lost their ctenidia, replacing them with lungs that can function in both water and air. Excretion of wastes is through structures called metanephridia and through the body and gill surfaces.
The blood circulates through the gill filaments, where exchange of carbon dioxide and oxygen occurs between the blood and the water flowing over the gill surface. Most molluskan blood contains a respiratory pigment called hemocyanin, a copper compound. When oxygenated such blood is bluish in color; when deoxygenated the blood is colorless. Only a few mollusks have hemoglobin in their blood. Blood circulation is variable within the phylum, but is generally mediated by a muscular heart, which distributes the blood to the tissues. Most mollusks possess well-developed sensory organs. The highest degree of development of the nervous system is found in the class Cephalopoda (octopuses, squids, and nautiluses).
Reproduction is sexual and may be simple or highly complex. The fertilized egg develops into a swimming form called a trochophore larva, which is seen also in the development of annelids; this then elongates to become a veliger larva, characteristic of mollusks, and differing in form in the different classes.