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UNIVALVES. 



Class I. — CEPHALOPODA. 



The Cephalopoda are decidedly the most perfect of the 

 Molluscs ; indeed, it may be asserted that in many re- 

 spects, they are more highly organised than the least 

 organised of the fish. The body is enclosed in a mantle, 

 which is in the form of a sack ; some kinds have a fin on each 

 side, others either an inner or outer shell, which is always 

 extremely symmetrical. The head is large, projecting from 

 the mantle, and bearing arms or feet as organs of motion, 

 arranged circularly round the mouth, whence the name of 

 the order. The mouth is armed with two very strong, 

 horny, partly calcareous jaws, which resemble the beak of 

 a parrot : the tongue is beset with numerous microscopic, 

 horny, barbed hooks, which are arranged lengthwise and 

 across, in the same manner as those of many Gasteropoda. 

 The crop is large and fleshy like that of a bird. The 

 breathing apparatus lying within the body consists of one 

 or two feathered gills on each side, and the heart lies 

 between the gills, and is divided into three parts ; so that, 

 in speaking of the Cephalopoda, it is not incorrect to speak 

 of three hearts. The eyes are very large and perfect, 

 generally concealed by a skin, which becomes transparent 

 at the place where it covers the eye ; it sometimes forms 

 folds which serve for eyelids. The organs of hearing con- 



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