684 ZOOLOGY SECT. 



prismatic layer (prs.) formed of minute prisms of calcium car- 

 bonate separated by thin layers of conchiolin ; and, lastly, forming 

 the internal part of the shell is the nacre (n.), or " mother-of-pearl," 

 formed of alternate layers of carbonate of lime and conchiolin 

 arranged parallel to the surface. The periostracum and the pris- 

 matic layer are secreted from the edge of the mantle only, the 

 pearly layer from the whole of its outer surface. The hinge- 

 ligament is continuous with the periostracum, and is to be looked 

 upon simply as a median uncalcified portion of the shell, which is 

 therefore, in strictness, a single continuous structure. 



By the removal of the shell the body of the animal (Fig. 563, B ) 

 is seen to be elongated from before backwards, narrow from side 

 to side, produced on each side into a mantle-lobe (m.) and con- 

 tinued ventrally into a keel-like visceral mass (Fig. 565, v.m.), which 

 passes below and in front into the foot (ft). Thus each valve of 

 the shell is in contact with the dorso-lateral region of the body 

 of its own side together with the corresponding mantle-lobe, and 

 it is from the epithelium (Fig. 564, ep. 1 ) covering these parts that 

 the shell is formed as a cuticular secretion. The whole space 

 between the two mantle-lobes, containing the gills, visceral mass, 

 and foot, is called the mantle-cavity. 



A single layer of epithelial cells covers the whole external sur- 

 face, i.e. the body proper, both surfaces of the mantle, the gills, 

 and foot ; that of the gills and of the inner surface of the mantle 

 (Fig. 564, ep. z ) is ciliated. Beneath the epidermis come connective 

 and muscular tissue, which occupy nearly the whole of the interior 

 of the body not taken up by the viscera, the coelome being, as we 

 shall see, much reduced. The muscles are all unstriped, and are 

 arranged in distinct bands or sheets, many of them very large and 

 conspicuous. The largest are the anterior and posterior adductors 

 (Figs. 563 and 565, a. ad., p. ad.), great cylindrical muscles, pass- 

 ing transversely across the body and inserted at either end into 

 the valves of the shell, which are approximated by their con- 

 traction. Two muscles of much smaller size pass from the shell 

 to the foot, which they serve to draw back ; they are the anterior 

 (a. r.) and posterior (p. r.) retractors of the foot. A third muscle 

 (prc.) arises from the shell, close to the anterior adductor, and has 

 its fibres spread fan-wise over the visceral mass, which it serves to 

 compress, thus forcing out the foot and acting as a protractor of 

 that organ. The substance of the foot itself consists of a complex 

 mass of fibres, the intrinsic muscles of the foot, many of which also 

 act as protractors. Lastly, all along the border of the mantle is a 

 row of delicate pallial muscles (Fig. 563, pi. m.), which, by their 

 insertion into the shell, give rise to the pallial line already seen. 



The coalome is reduced to a single ovoidal chamber, the peri- 

 cardium (Fig. 565. pc.}, lying in the dorsal region of the body and 

 containing the heart and part of the intestine ; it is lined by 



