268 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the dorsal side of the body, known as the mantle, where it is greatly 

 thickened and calcified, forming the characteristic shell. Correlated 

 with the presence of the rigid shell dorsally this portion of the body-wall 

 has little development of muscle. Ventrally on the other hand there is 

 a great development of muscle, forming one of the most characteristic 

 organs of the mollusc — the foot (Fig. in, F). 



The shell is composed typically of three distinct layers. Externally 

 is an uncalcified layer — the periostraeum — well seen in fresh- water 

 mussels. This uncalcified cuticle in the mollusc is composed not of 

 ordinary chitin but of a somewhat different substance — conchiolin, 

 resembling silk in its composition. Next to the periostraeum is the 

 thickest layer — the prismatic layer — composed of practically pure calcium 

 salts (mainly carbonate) arranged in minute prisms perpendicular to the 

 surface of the shell. Lastly and internally is the thin nacreous layer 

 composed of the same calcium salts deposited in very fine sheets, one 

 over the other. These sheets cropping out all over the surface form 

 extremely minute ridges which cause interference between the rays of 

 light reflected from the surface and so produce in many cases the beautiful 

 iridescence which gives this layer its popular name " mother-of-pearl." 

 As the mollusc grows the shell is enlarged round its edge by the mantle 

 adding on to the periostraeum and prismatic layers, while additional 

 deposits of nacreous layer are formed by the activity of the mantle all 

 over the inner surface of the shell. Occasionally small pockets are 

 formed in the outer surface of the mantle, e.g. round some intruding 

 parasite such as a larval Trematode or Cestode, and if such a pocket 

 becomes closed in its cavity becomes filled with concentric layers of 

 nacre forming an isolated rounded mass — a pearl. Pearls are liable to 

 occur in many different molluscs, mostly Pelecypoda, including the 

 common mussels (Mytilus) and fresh-water mussels (Anodonta, Unio), 

 but the finest and most abundant specimens are obtained from the 

 Pearl oyster of tropical seas {Avicula). 



The shell shows great variety of form, the most primitive probably 

 being a simple shallow dome covering the dorsal surface. In two of the 

 main groups — ^the Gasteropoda and the Siphonopoda — ^this dorsal surface 

 bulges out greatly, forming the visceral hump which contains most of 

 the internal organs of the body. To accommodate this the shell is drawn 

 out into a long cone, and to avoid unwieldiness the cone grows in such 

 a lopsided manner as to form a tightly coiled (usually right-handed) 

 spiral which may be anything between perfectly flat, as in some of our 

 common fresh-water snails (Planorbis), and a long slender tapering screw 

 (Turritella). In some cases the visceral hump does not continue to fill 



