Circulatory system
The heart lies within the pericardium which represents part of the much reduced coelom or body cavity. It consists of a single muscular ventricle into which open usually one pair, but in a few cases two pairs, of auricles. Contractions of the ventricle force blood into anterior and posterior aortae; it then circulates round the body in sinuses (only the highly organised cephalopods have capillaries), passing through the kidneys and finally returning to the auricles by way of the ctenidia. The blood is either colourless or contains the blue respiratory pigment haemocyanin. Blood is extensively used in molluscs for hydrostatic purposes as well as for respiration: it forces the body out of the shell, extends the foot, and so on.
Alimentary system
The mouth, which may bear ‘jaws’, lies at the anterior end of the head. Just within the cavity and capable of being protruded is the radula or lingual ribbon. This typically molluscan structure, lacking only in the bivalves, is a horny ribbon which bears rows of teeth and is capable of wide modification for scraping, piercing, seizing or even poisoning prey. It also acts as a conveyor belt carrying food into the gut. It is continually being worn away but at the same time being added to in the radular sac whence it originates. It is carried over a muscular pad known as the odontophore and lubricated by mucus from salivary glands which in some cases may be modified to produce digestive enzymes or even poison. Digestive glands may occur in the oesophagus leading to the stomach into which open a mass of blind-ended tubules or digestive diver- ticula. Universally present in molluscs, these tubules are concerned, in varying degrees in different groups, with intracellular (i.e. phagocytic) digestion, absorption, secretion of digestive enzymes, excretion and, in some gastropods, with storage of the calcium needed for the formation of the shell.
The stomach itself is a largely ciliated organ broadly concerned with sorting material into finer particles which pass into the tubules of the digestive diverticula and larger ones which pass into the mid gut or intestine for consolidation into faecal pellets. Varying extents of the stomach wall are covered with a translucent, firm gastric shield. This is frequently in functional association with the crystalline style, a rod-like structure formed from globulin (a protein). This is secreted in a style-sac which forms the posterior extension of the stomach. Cilia lining the sac cause the style to rotate. The head of the revolving style assists in drawing food into the stomach and in its mixing while, as the style slowly dissolves, it liberates digestive enzymes which break down sugars, and even in some cases cellulose, and also fats. Universally present in the bivalves, this remarkable structure (possibly the only revolving structure in animals) also occurs in the monoplacophoran Neopilina and in many gastropods.
The anus opens into the mantle cavity. In association with this the mid and hind gut (or rectum) are primarily concerned with consolida- tion of faeces which would foul the respiratory chamber. The anus opens above the ctenidia and the faeces are carried away in the exhalant stream.