STRUCTURE AND HABITS. 63 



of moUusks, of which the typical structure is that of a long 

 and conical mass^ partly contained in an enlarged spiral 

 shelly wliich protects the soft and visceral portion ; partly 

 protruding, in its broad extremity, from the aperture of the 

 shell, and comprising the head and mantle, the nerves and 

 gills and muscles, with a broad, fleshy, muscular, expansile 

 disc, mostly attached to the neck, which acts the part of a 

 foot, and alternately contracts and dilates. 



These creatures are extremely numerous, and of consi- 

 derable interest. They present . extraordinary varieties of 

 form and colour; of sculpture also, for their domes are 

 elaborately carved and adorned ; and their diversity of habit 

 is equally remarkable. The greater number are marine, 

 and yet colonies abound in lakes, and stagnant pools, and 

 rivers, dwelling also on land or climbing trees ; and as their 

 breathing apparatus is necessarily adapted to the places they 

 inhabit, the respiratory organs, together with certain modi- 

 fications in their arrangement, are selected as characters, 

 whereby to distribute the class into orders. 



Such is the merest outline of this interesting class. Let 

 us consider the further peculiarities of their structure. 



In Gastropods, the head forms the outer extremity, and is 

 mostly rounded and prominent ; it possesses from two to 



