414 MANUAL OF ZOOLOGY. 



The Heteropoda are pelagic in their habits, and are found 

 swimming at the surface of the sea. They are to be regarded 

 as the most highly organised of all the Gasteropoda, at the 

 same time that they are not the most typical members of the 

 class. Some of them can retire completely within their shells, 

 closing them with an operculum ; but most have large bodies, 

 and the shell is either small (fig. 220) or entirely wanting. 





Fig. 220. Heteropoda. Carinaria cymbium. p Proboscis; ^Tentacles; <5 Branchiae ; 

 s Shell ; / Foot ; d Disc. (After Woodward. ) 



They swim by means of a flattened ventral fin, or by an elon- 

 gated tail, and adhere at pleasure to sea -weed by a small 

 sucker situated on the side of the fin. These organs are 

 merely modifications of the foot of the ordinary Gasteropods ; 

 the fin-like tail being the " metapodium " (as shown by its 

 occasionally carrying an operculum), the sucker being the 

 " mesopodium," and the ventral fin being a modified " pro- 

 podium." The " epipodia " are apparently altogether wanting. 

 Respiration is sometimes carried on by distinct branchiae, but 

 in many cases these are wanting, and the function is performed 

 simply by the walls of the pallial chamber. 



The Heteropoda are divided into the two families Firolida 

 and Atlantida, the former characterised by having a small 

 shell covering the circulatory and respiratory organs, or by 

 having no shell at all; whilst in the latter there is a well- 

 developed shell, into which the animal can retire, and an oper- 

 culum is often present. 



SUB -CLASS B. PULMONIFERA Or PULMOGASTEROPODA. In 



this sub-class of the Gasteropoda respiration is aerial, and is 

 carried on by an inflection of the mantle, forming a pulmonary 

 chamber into which air is admitted by an external aperture. 

 The flexure of the intestine is nettral, and the sexes are united 

 in the same individual. 



The Pulmonifera include the ordinary Land-snails, Slugs, 



