CHAPTEE III. 



POtYZOA MOLLUSCA GASTEROPODA BRACHIOPODA CONCHIFERA 

 CEPHALOPODA PTEROPODA TUNICAT A CRUSTACEA EN TOMOSTBACA 

 ANN ULOS A CIBRIPEDA E NTOZOA ANNELI l> A . 



HE term Mollusc, derived from mollis, 

 soft, is one which at once indicates a 

 chief structural characteristic of the 

 class of animals about to occupy our atten- 

 tion. The body of most of the molluscous 

 sub-kingdom is soft and fleshy ; and all except 

 the Tunicata and a few Pteropoda are covered 

 or protected by a hard calcareous shell. The 

 shell is of two kinds ; first of an epidermal 

 character, being formed upon the surface of a 

 filmy cloak-like organ called a mantle, an- 

 swering to the true skin of othei animals ; 

 and next of a dermal character, being con- 

 cealed within the substance of the mantle, 

 and frequently moulded into a great diversity 

 of forms, and coloured with various tints. 



The molluscs belonging to the class Gaste- 

 ropoda have become a large and important 

 section of the animal series, presenting very 

 'I many objects of great interest for the micro- 

 scopist. Of the large family of molluscs, the 

 only species which bears any resemblance in structure to 

 the Polyzoa, which now form a portion of the molluscous 

 division, is the Brachiopoda, and this is confined to a re- 

 semblance in internal structure. 



