MOLLUSCA 



155 



Fig. 86.— Chiton. (From 

 Kingeley, after Haller.) 



may split transversely into eight valves, or it may re- 

 Lain in one piece (Fig. 86). If it remains in one 

 piece it may remain flat, or it may 

 become tubular or coiled in a spiral 

 (Fig. 87). As the mantle is con- 

 tinually active, and the animal 

 constantly growing larger, the new 

 layer of shell always extends a lit- 

 tle beyond the old layer. The 

 oldest part of the shell is thus 

 always the thickest and each new 

 layer is indicated by a line of 

 growth. The shell is a very per- 

 fect reproduction of the mantle. 

 Even colored spots and outgrowths from its surface 

 reappear on the surface of the shell. 



The Clam. — In a common mollusc like the clam (Fig. 

 88) the body is sac-like and is bilaterally symmetrical. 

 It is divided into two parts: an upper or dorsal part 

 which contains the internal organs (Fig. 89), and a 

 lower, muscular part called the foot which controls loco- 

 motion. The mantle is attached to the dorsal, or upper, 

 edge of the body and hangs down on each side of the 

 body. Its edges may be entirely separated, or they may 

 be united in such a way that three openings are formed. 

 Through the largest of these the large muscular foot 

 passes ; through the other two, currents of water. The 

 two smaller openings, called siphons, are guarded by a 

 circlet of cilia whose motion causes a current. Through 

 one water enters carrying food and oxygen ; through the 

 other comes out water carrying waste. These siphons 



