464 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



The Higher Crustaceans are equally incased in 

 an unyielding shell more calcified than that of insects; 

 and yet they grow. This is possible, however, only by a 

 periodic shedding of the shell, which leaves them for a 

 time almost helpless for want of a rigid skeleton until a 

 new one is formed by deposit of carbonate of lime in the 

 skin. 



In lower crustaceans the shell is chitinous, like that 

 of insects. Thus crustaceans have been divided by their 

 shell substance into a lower group (Entomostraca), in- 

 sect-shelled or chitinous-shelled, and a higher group 

 (Malacostraca), mollusk-shelled or calcareous-shelled. 



MOLLUSKS. 



These are par excellence shell-covered animals, and 

 their classification is largely based on the character of 

 their shells. 



Acephala have two shells, a right and a left, hinged 

 along the back — bivalves. Gastropods have but one, usu- 

 ally much coiled shell — 

 univalves. Cephalopods, 

 when they have a shell 

 at all, are distinguished 

 by their many-chambered 

 structure. The animal 

 lives only in the large 

 outer chamber. All the 

 other chambers are 

 closed and filled with 

 air, and a slender mem- 

 branous tube — the siphuncle — runs from the animal 

 through all these chambers, but not opening into them 

 ( Fi g- 325)- 



Growth of Shell.--The shell is a calcareous secre- 

 tion by the skin of the mantle. In bivalves (see Figs. 280 



Fig. 323. — Surface view of a bivalve, 

 showing lines of growth or successive 

 sizes of the shell. 



