266 
expanded, is quite large. By the contraction of the muscles, as de- 
scribed above, the mantle chamber is almost obliterated, and the water 
that it contained is forced through the only channel left it, the 
posterior opening. When the muscles relax and the foot is protruded, 
the elastic cuticle expands the mantle chamber to its greatest capacity, 
and the animal is ready to repeat the process. 
The jets formed in this way follow each other rapidly, and, as 
each jet will send an animal more than its length, swimming is quite 
rapid. Animals frequently swim some feet before settling to the 
bottom. 
Most Lamellibranchs possess the power of sending jets of water 
from the mantle chamber, but they generally depend entirely upon 
forcibly shutting the shell. 
Pecten and other forms swim by this means, but in most cases 
it seems to serve principally to rid the mantle chamber of dirt and 
irritating substance. It also serves to cause a more thorough exchange 
of water but in most cases, where tides and currents exist, this must 
be of rather secondary importance. For forms that live in burrows 
or crevices it may be of much greater importance, as the water sur- 
rounding the animal would soon become stale and barren of food 
without some means of changing it, and the ordinary currents formed 
by cilia might not be sufficient. Other forms such as the soft shelled 
clam, Mya arenaria, have the power of forcing water from the mantle 
chamber greatly developed, and depend upon the contraction of muscles 
in the mantle as well as on closing the shell. Without some such 
means of getting rid of dirt that gains access to the mantle chamber, 
such forms, living deeply buried in rather soft mud, in shallow water, 
and between tides, where currents caused by tides and waves are 
continually stirring up the mud, would become clogged in a very short 
time. 
Until something more is known about the life and feeding habits 
of Solenomya, we can hardly expect to know the full significance of 
its mode of expelling water. Very possibly the jets are of value in 
cleaning the mantle chamber and burrow, and the animal has made 
use of them secondarily as a means of locomotion. 
