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 the 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 Lamelli- 
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 (Anomia). 
The body of the Lamellibranch lies suspended within the man- 
tle-cavitv, being attached to the mantle along its dorsal edge. The 
mouth is near the dorsal side of the anterior end of the body, away 
from 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 byssus. 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 cavity. Indeed the name Lamellibranchiata means 
