MOLLUSCA. 251 



clam or mussel. Illustrations should be found in the advanced text-books 

 and presented to the class. 



294. Ecology. The bivalves are sedentary or sluggish in 

 their manner of life; the motion of most of the gasteropods 

 is slow and difficult. In conformity with their limited powers 

 of locomotion, they are scavengers, feeding on the debris and 

 the small animals and plants brought to them by the water 

 currents (oysters, mussels, etc.), or are largely herbivorous 

 (many snails). A very few are parasitic. The cephalopods 

 are much more active and are carnivorous. For the most part 

 the sluggish forms are well protected by the shells, neverthe- 

 less they furnish food for many diverse sorts of animals. 

 Some of their enemies are internal parasites, others bore 

 through the shells and thus gain access to vital parts. 



The animal within may thwart this attack of its enemies by 

 the continued secretion of mother-of-pearl on the inner sur- 

 face at the threatened point. Some animals crush the shells, 

 or swallow the mollusk, shell and all. Star-fishes, as we have 

 seen, are especially troublesome to the oyster beds. 



Many of the bivalves are capable of still further protection 

 because of their elongated siphons which enable them to 

 burrow deeply in the mud or sand, the food being carried in 

 through the siphons by the water currents (Fig. 1 14). Several 

 species of marine bivalves have the power of boring into wood 

 or even stone. This serves as a protection to them, but often 

 results in the complete destruction of piles and other structures 

 placed in the ocean by man. 



Many of the mollusks seem more or less gregarious, as is 

 illustrated by beds of clams and oysters, the schools of squid, 

 etc. 



Notwithstanding the low organization and sluggishness of 

 a large portion of the branch Mollusca, we are compelled to 

 consider that it has been a very successful group in that it 

 has held its place with practically equal numbers through the 

 geological ages, and has succeeded in adapting itself to the 

 changes of those ages. Of no less interest is the additional 



