118 SEA-SHORE LIFE 



tended as a long double tube, erroneously called the " neck," that 

 projects outward from the posterior end of the shell. This neck 

 should be called the siphon and if one observes a living clam it 

 will be seen that a constant current of water is passing in through 

 the opening in the siphon that is farther away from the hinge, and 

 pouring out through the one nearer the hinge side This current is 

 caused by the beating in unison of myriads of cilia that cover the 

 gills of the clam; and thus water is brought in to aerate the blood, 

 and to provide the minute organisms upon which tlie clam feeds, 

 while the waste water and products of excretion are carried away 

 through the dorsal-most opening. But the mantle serves not only 

 to provide definite openings for water currents. It secretes the 

 horny outer skin, and the inner stony layers of the shell. 



In all of the young and in the great majority of adult I;amelli- 

 branchs the two valves of the shell are exactly alike in shape, but 

 in those which live attached to objects the valves are often dissimi- 

 lar, as in the case of the oyster and the jingle shell fAnouiiaJ. 



The body of the Tvaraellibranch lies suspended within the man- 

 tle-cavity, being attached to the mantle along its dorsal edge. The 

 mouth is near the dorsal side of the anterior end of the body, away 

 frmn the siphon. It has no teeth, and is a deep groove bordered 

 above and below by projecting ridges which function as lips. 



The foot is a muscular expansion on the ventral side of the 

 body. In some attached forms the foot is very degenerate, but in 

 many of the clams it is developed into a strong blade-like organ, 

 capable of great expansion and contraction, and serving to move the 

 animal from place to place, to burrow, and in some forms even to 

 swim. In many forms the foot is provided with a special gland 

 that secretes a glue-like substance which adheres to anything it 

 touches, and hardens into a tough, elastic thread serving to fasten 

 the mollusk to an anchorage. This thread or rather accumulation 

 of threads is called the hy.ssns. The byssus may usually be cast off 

 at will, and renewed thread by thread. By means of these threads, 

 the mussels are able to drag themselves slowly about, or even to 

 climb. 



The most characteristic organs of Lamellibranchs are the sheet- 

 like gills that arise from the sides of the body, and hang freely 

 within the mantle cavitv. Indeed thennme TxtiiieUih)'anc]iiata means 



