//. TYPES IN DETAIL U^ITH THEIR SUBORDINATE GROUPS. 19 



shell, usually capable of receiving 

 the other parts when retracted; 

 mouth furnished with a radula; 

 foot forming a creeping disc. 



Order i. Prosobranchiata .... Shell present; gill anterior to 



heart; foot a creeping disc; sexes 

 separate. 



X Oliva, Cyprcea, Strombus. 



Order 2. Heteropoda Shell small or wanting; gill as in 



previous order; anterior part of foot 

 compressed, forming a sort of keel ; 

 sexes separate. This order in- 

 cludes a very few nearly transparent 

 forms, which swim on the surface 

 of the ocean. x Carinaria. 



Order 3. Pulmonata Land and fresh- water snails breath- 

 ing by plexus of blood-vessels 

 which lie in a respiratory chamber 

 communicating with the exterior, 

 and placed anterior to the heart; 

 shell generally present; hermaph- 

 roditic. X Helix, Limnaa. 



Order 4. Pteropoda Shell fragile or wanting, foot de- 

 veloped into a pair of wing-like 

 expansions, hermaphroditic. A 

 small group of forms which swim 

 at night upon the surface of the 

 ocean. x Cymbuliopsis. 



Orders. Opisthobranchiata. .. Shell, when present, rather deli- 

 cate, often wanting; gill placed 

 behind heart ; back of naked forms 

 often ornamented with simple or 

 dendritic papilte ; hermaphroditic. 

 X Bulla, Eolis. 



Class V. CEPHALOPODA Head well developed, distinct from 



body and with two large eyes; 

 mouth surrounded by many arms 



