THE PHYLUM MOLLUSCA 



ever, even in regions near the umbo, is successfully repaired by the secretory 

 activities of the underlying mantle surface. 



Pearls. The formation of a pearl by a bivalve is an abnormal manifesta- 

 tion of its shell-forming activities. Any small foreign body, such as a sand 

 grain or a parasite, coming to lie between the mantle and the valve, is 

 enfolded by the mantle in a small invaginated pocket. Copious secretion of 

 nacre by the epithelium of the pocket follows, and the foreign particle be- 

 comes coated with successive layers of this material. Many years may be 

 required for the growth of a large pearl. In Japan, a flourishing industry 

 has grown from the discovery that a small piece of shell inserted into a 

 pocket of the mantle in the pearl oyster, Pmdada, will in the course of 

 several years be transformed into a commercially valuable "cultured" pearl. 

 Oysters bearing such implanted seeds are maintained in wire cages and care- 

 fully tended to insure a maximal rate of growth. Pearls are found occasionally 

 in fresh-water mussels, but they are generally irregular in shape and of 

 inferior color. 



Other Pelecypoda. Although the bivalved shell can be recognized with- 

 out difficulty in almost all species, there is great diversity in the super- 

 ficial appearance of pelecypods. Interesting differences are shown, for ex- 

 ample, with respect to the relative sizes and positions of anterior and posterior 

 adductor muscles. In such forms as Venus mercenana and the fresh-water 

 mussels, the two adductors are about equally developed; this is considered the 

 primitive condition. In another group, exemplified by the marine mussel, 

 Mylilus, the anterior adductor is much smaller than the posterior, and a line 

 drawn between the two muscles would lie at an angle to the hinge axis. In 

 such forms as the oyster, Crassostrea, and the scallop, Pecten, the anterior ad- 

 ductor is lacking, and only the posterior adductor remains to close the 

 valves (Figs. 13.4, 13.7). 



Other variable characteristics include the size and weight of the shell and 

 the size and development of the foot and siphons. In sedentary species such 

 as the oyster, the foot is very much reduced, and in such burrowing forms as 

 Emis, the razor clam, and Mya, the "steamer," the foot is very large (Fig. 

 13.8). Adaptation to the habit of burrowing deeply into mud or peat, into 

 rock, and into wooden structures exposed to salt water commonly involves 

 reduction and modification of the shell and increase in the size and length of 



Fig. 13.7. Shells of bivalve 

 mollusks, showing attachments 

 and relative sizes of anterior 

 and posterior adductor mus- 

 cles. A, a fresh-water mussel; 

 B, marine mussel, Mylilus: C\ 

 oyster, Crassostrea, which in the 

 adult sta8;e lacks an anterior 

 adductor muscle. 



379 



