INVERTEBRATE ZOOLOGY. 41 



Class III. SCAPHOPODA Bilaterally-sj'mmetrical forms with body 



cavity greatly elongated in a dorso-ven- 

 tral direction. Mantle and shell tubular 

 and somewhat curved, with a smaller 

 dorsal and a larger ventral opening. 

 Ctenidia fail. Foot elongated and coni- 

 cal. A single familj' of marine forms. 

 •if Dentalium. 



Class IV. Gasteropoda Molluscs with a head, foot and visceral 



sack. The first two are bilateral, the 

 third is almost invariably unsymmetri- 

 cal, the pallial complex, with its organs 

 being developed upon one side only (usu- 

 ally the right). The visceral sack is 

 generally contained in a spirally twisted 

 shell, wound usiaally toward the right 

 aboiit a central axis, and capable of re- 

 cei\ang the other parts when contracted. 

 The mouth is furnished with a radula : 

 the foot generality forms a creeping disc. 



Order 1. Prosobranchiata Shell present, ctenidium anterior to 



heart ; foot a creeping disc ; bisexual. 

 ifOliva. Conns, Cyiyraea, Stromhus. 



Order 2. Heteropoda Shell small or wanting. Gill as in pre- 

 vious order. Anterior part of foot com- 

 pressed, forming a sort of keel. Bisex- 

 ual. This order includes a very few 

 nearly transparent forms, which swim on 

 the siirface of the ocean. They may be 

 considered as Prosobranchiata adapted to 

 a pelagic life. ^^ Carinaria. 



Order 'i. Pulmonata Land and fresh water snails, breathing 



by plexus of blood vessels, which lie in 

 a respiratory chamber communicating 

 with the exterior, and placed anterior to 

 the heart. Ctenidia fail. Shell generally 

 present. Hermaphroditic. 

 ^ Helix, Limnaea, 



