THE ACEPHALOUS MOLLUSKS. 187 
organ, which is, from one of its uses, called a foot; but it resembles 
rather a large tongue. In the many drawings of these gastero- 
pods, its varied shapes are shown. In some species the tissues 
of which this organ is composed are spongy, and are capable of 
being filled with water, and so the creature is enabled to extend 
and stiffen the foot; by expelling the water, the limb returns to 
its limp condition, and resumes its place within the shell. 
Mollusks use their foot very cleverly. They stretch it out, 
TELLINA PULCHERRIMA. 
fix it by its point, then contract it, and so pull themselves 
along. Réaumur compared the advance of these creatures to a 
man who lies down on the ground, stretching forward his arms, 
seizes some solid body before him, and then draws himself to it; 
MODIOLA LITHOPHAGA, 
the only difference being that, in the case of the mollusk, the 
member does not double itself up in joints, but contracts in its 
whole length. 
In some few and rare instances, the mode of progression is 
exactly the reverse: the creature plants the point of its foot 
firmly against the sand, and then stiffens it, thus pushing itself 
forward, just as a bargeman urges on his boat by thrusting his 
pole against the bottom of the river. There are acephalas which 
progress by leaps. This peculiar species of locomotion is effected 
by a sudden opening and shutting of the valves. The Pectens 
sometimes dart through the waves to avoid a danger, and the 
