LAMELLIBRANCHIATA. 485 



dorsal but uncalcified region of the shell. Calcareous interlocking and 

 asymmetrical processes of each valve, placed at the exterior 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 Cardium, 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 (Monomyd], 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 



