192 
THE MOLLUSKS 
animal thus far considered. The mouth of cephalopods 
is surrounded with tentacles. 
A common squid, Sepia (se'pi-a), has ten arms or tentacles, 
two long and eight short. It moves itself forward rapidly 
by shooting out water from a siphon in the collar region. 
When pursued, the squid ejects an ink-like fluid which 
clouds the water, concealing it from its prey and facilitating 
its escape. 
It, like all other animals, begins as a single cell. 
Cuttle fishes are similar to squids, the marked differences 
being in the shape of fins, the form of the eyes, and the 
shape of the longer tentacles. 
The octopi are the largest members of the group. They 
have eight tentacles, which in some cases reach a length of 
thirty feet. The stories about the size and behavior of the 
octopi are often exaggerated. 
182 . Economic Importance of the Group. — Clams, 
scallops, oysters, and snails are used as food in all parts of 
the world. In this country, oysters are gathered in great 
abundance from Chesapeake Bay and other bays along the 
Atlantic Coast. 
The edible clams are of two kinds. The round clam, 
Venus mercenaria (Ve'nus mer-se-na'ri-a), is more generally 
