320 THE OCEAN WORLD, 
it is elongated and stiff; on the other hand, by suddenly expelling 
all the water, it gets small and pliable, and can now return to its 
shell. This organ is represented in Fig. 127 (Donax trunculis, 
Linn.), in which it is singularly well developed. This bivalve is 
found on the sea-shore in shallow water; it buries itself almost per- 
pendicularly in the sands. They are so abundant on the French 
side of the Channel and on the shores of the Mediterranean, that 
they form a considerable portion of the people’s food. These bivalves 
have the singular power of leaping to a considerable height and 
then throwing themselves to a distance of ten or twelve inches—a 
spectacle which may be witnessed any day at low water. When 
abandoned by the retreating tide, they try to regain the sea. If 
seized by the hand, in order to drag them out of the sand, aided by 
their compressed, branched, and angular feet, they give to their shell 
Fig. 127.—Donax trunculus (Linnzus). 
the sudden and energetic movement under which the bounding action 
takes place. The shell of the genus Donax is slightly triangular and 
compressed ; its length exceeds its height; it is regular, univalve, 
unequally lateral, and its hinge bears three or four teeth on each 
valve. The action of these feet is very simple, and is compared by 
Réaumur to that of a man placed on his belly, who, stretching out 
one hand, seizes upon some fixed object, and draws himself towards 
it. There is just this difference, that the movement of the member 
in the mollusc is altogether contractile. 
Authors have described more than 4,000 species of bivalve mol- 
luses, so that our space only permits us to describe a few families, or 
rather types of families. 
The arrangement of bivalves now most generally adopted in 
England is that of the late Mr. Woodward, as developed in the last 
edition of his manual of the mollusca ; it is greatly based on that 
of Lamarck. We have adopted his arrangement altered from a 
descending to an ascending scale of organisation. 
The Lamellebranchiata, so called from the leaf-like form assumed 
