224 ELEMENTARY ZOOLOGY. 



and attack the soft parts of other mollusks; starfish are 

 the greatest enemy of the oyster, and probably of other 

 attached or slow moving kinds; fishes, frogs, and birds 

 all help in their destruction. 



The shell of course is a means of protection against 

 these enemies. These shells may be colored on the out- 

 side in such a way as to be inconspicuous, or minute 

 growths of various kinds may cover them in such a way 

 as to secure the same end. A good many of the lamelli- 

 branchs become attached or fixed in one way or another. 

 The poor development of the nervous system and the 

 general appearance of degeneracy, as compared with 

 the other mollusks, is doubtless correlated with this fact. 

 Some bore into wood or even stone when they first settle 

 down, and burrow out a chamber in the solid object, 

 which increases as they grow in such a way that they 

 cannot get out through the opening. Find figures of the 

 ship-worm (Teredo) or of Pholas or some other rock-bor- 

 ing type. The long-necked clam burrows in the sand 

 (Fig. 72). The burrowing forms have the siphon, a 

 special tubular growth of the mantle, through which they 

 reach out into the water and get their supplies. The 

 oyster attaches itself permanently to the rocky bottom 

 by one valve. 



The cephalopods find safety in their splendid powers of 

 swimming and in their ability to fight. 



246. Relation to Human Interest. This is the first 

 group that we have studied that has any value, worthy 

 of mention, as a food to man. The oyster industry is a 

 matter of considerable magnitude, though the actual 

 food contained in oysters is slight. It is estimated that 

 about ten million bushels of oysters are marketed annu- 



