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OF CRAWLING MOLLUSCANS. 



Class I. GASTEROPODA Cuv. 



The animal is furnished with a distinct head, two or 

 four tentacles, and a broad expanded foot for locomo- 

 tion; and is generally protected by a straight, oblique, 

 or spirally-twisted conical univalve shell. This class is 

 divided into orders, according to the form of their re- 

 spiratory organs ; thus : — 



A. The gills comb-like, formed of a ridge of plates or 

 filaments, on the inner side of the mantle, over the 



back of the neck. (Ptenobranchiata.) 



I. Zoophaga, the edge of the mantle produced into 

 a siphon. 



II. Phytophaga, the edge of the mantle simple. 



B. The gills variable, arborescent, or the respiratory 

 organs in the form of lungs. 



III. Pleurobranchiata, the gills on the side of the 

 body, under the edge of the mantle. 



IV. Gymnobranchiata, the gills naked on the back, 

 or round the inner edge of the mantle. 



V. Pneumobranchiata, the respiratory organs con- 

 sisting of a bag formed by the mantle, and lined 

 with the pulmonary vessels. 



e 3 



