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 



.;tile development of muscle. Ventrally on the other hand there is 



at development of muscle, forming one of the most characteristic 



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



The shell is composed typically of three distinct layers. Externally 



, uncalrified layer the periostracum well seen in fresh-water 



This uncalcified cuticle in the mollusc is composed not of 



ordinary chitin but of a somewhat different substance eonehiolin, 



iMing silk in its composition. Next to the periostracum is the 



thickest layer the prismatic layer composed of practically pure calcium 



i 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 



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



mely 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." 



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



adding on to the periostracum 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 



i site such as a larval Trematode or Cestode, and if such a pocket 



lose.l in its cavity becomes filled with concentric layers of 



nitu: an isolated rounded mass a pearl. Pearls are liable to 



occur in many different molluscs, mostly Pelecypoda, including the 



non mussels (Mytilus) and fresh-water mussels (Anodonta, Unto), 



but the finest and most abundant specimens are obtained from the 



I' 1 ->i i ! tropical seas (Avicula). 



hell shows great variety of form, the most primitive probably 



ni pie shallow dome covering the dorsal surface. In two of the 



mam -roups il u - Casteropoda and the Siphonopoda this dorsal surface 



rming the visceral hump which contains most of 



tin- internal organ* of the body. To accommodate this the shell is drawn 



< . and to avoid unwieldiness the cone grows in such 



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



I' may I..- anything between perfectly flat, as in some of our 



ah water snails (Planorbis), and a Ion- slender tapering screw 



ases the visceral hump does not continue to fill 



