Mollusca with rudimentary prostomium. Eyes absent on the prosto-mial region of the adult. No odontophore or jaws.
Lipocephala with very well-marked bilateral symmetry. There are two large mantle-folds, a right and left, each covered by one of the valves of the shell. An elastic ligament unites the two valves of the shell together in the median dorsal line. The foot is laterally compressed and usually sharp-edged. There are two pairs of labial tentacles. The gill-filaments usually undergo concrescence and form gill-lamellae. There are three pairs of ganglia, the cerebro-pleural, pedal, and visceral. The liver, auricles, nephridia, sexual glands are paired and symmetrical. Salivary glands and accessory organs of generation are wanting.
The valves of the shell are usually equivalve, but rarely equilateral as in some species of Pecten. Accessory pieces may be present in the median dorsal line as in Pholas. The valves may be small and cover the mantle-surface incompletely, and then the animal clothes its burrow in wood (Teredo) or sand (Septarid) with a calcareous lining. Or though free originally, they become fused at an early period into a continuous calcareous tube secreted by the general surface of the mantle, open posteriorly, and often anteriorly as in Aspergillum and its allies (Gastrochae-nidae). In Ostreidae the right valve is firmly fixed to stones. The shell is often produced into processes or ridges formed by lobes of the mantle-edge; it is generally coloured by pigments derived from unicellular glands in the same region. The ligament must be regarded as a median dorsal but uncalcified region of the shell. Calcareous interlocking and asymmetrical processes of each valve, placed at the-anterior end of the ligament, and known as hinge- and cardinal-teeth, unite the valves in many forms (see p. 125). The two mantle flaps or folds are often united indirectly at the posterior end of the ctenidial axis (p. 128); sometimes also directly for a greater or less extent of their ventral edges.
The margin of each fold is thickened, muscular, and contractile, and often carries a number of tentacles on its inner surface. Its posterior region is often prolonged into two tubes or siphons, which may be of great extent and either free from one another for a greater or less extent, or united almost to their extremities. Of these siphons, the ventral is inhalent, the dorsal exhalent, the currents being caused by the action of cilia on the inner surface of the mantle, on the ctenidia and labial tentacles.
The foot, the sole organ of locomotion, is lost or rudimentary in the Ostreidae and some other families. It is typically ploughshare-shaped, and is used for burrowing in sand or mud, rarely disc-shaped and used for crawling as in Arcadae. It is bent upon itself and can be used for jumping in Cardiutn, Trigonia, and in Solen and its allies is a long cylindrical burrowing organ, which when rapidly contracted causes water to spurt out of the siphons and so propels the animal in the water. The Pectinidae swim by alternately opening and shutting the valves of the shell. The edge of the foot often contains a special gland, the byssus gland, which secretes a horny material in the form of filaments, by means of which the animal is attached to foreign objects. The gland is sometimes present in a rudimentary condition in the young animal, though aborted in the adult, e. g. Anodon.
The musculature of the body forms certain special muscles. These are, the muscles that close the valves of the shell or adductors; those that move the foot, protractor, anterior and posterior retractors; and the pallial muscles. The adductors are formed by transverse fibres, which pass from one to the other valve. In many forms there is one, the anterior, in the prostomial region, and a second, the posterior, at the hind end of the body, the former above the mouth, the latter below the anus. The anterior adductor may be absent altogether (Monomya), but may then be present in the embryo as in Ostrea. The protractor and anterior retractor of the foot are inserted close to the insertion of the anterior adductor, the posterior retractor near the posterior adductor (p. 125). They are paired, one on each side of the body, but the posterior retractors are united for a certain distance. The pallial muscles are retractors and compressors of the free edge of the mantle. The insertions of these muscles are the cause of 'impressions' on the inner surface of the valves.
The impression of the pallial muscles, termed 'pallial line,' either follows the contour of the edge of the shell in its whole extent, or when the siphons are large curves inwards forming a deep bay at the posterior end of the valves. These two conditions are known respectively as integro- or sinu-palliate.
The cerebro-pleural ganglia usually lie at the sides of the mouth connected by a dorsal commissure. They are sometimes fused dorsally (Teredo). The pedal ganglia are always closely united. Their size varies with the development of the foot and they are absent in the Ostreidae, but an infra-oesophageal cord passes from one cerebro-pleural ganglion to the other and thus represents the pedal system. The pedal connectives are sometimes excessively short and the ganglia approximated to the cerebro-pleural as in Pecten. The visceral ganglia are always large, their connectives long, and the two are closely united. They lie on the ventral surface of the posterior adductor muscle, and give off branches to the viscera. The two branches to the ctenidia are each connected with an osphradial ganglion, and when the siphons are large, e.g. Solen, Mya, there are siphonal ganglia developed on the siphonal nerves. Both cerebro-pleural and visceral ganglia give off pallial nerves, which traverse the margins of the mantle and may unite, e. g. Ostrea, to form a circumpallial nerve, or are resolved into a circumpallial plexus, e.g. Anodon. Nerve and plexus alike contain ganglionic centres.
The chief ganglia are often orange coloured.