MOLLUSCA: GASTEROPODA. 293 



motion effected by a fin-like tail, or by a fan-shaped, vertically- 

 flattened, ventral fin. 



Fig. 105. Heteropoda. Carinariacyntbium. ^Proboscis; t Tentacles; b Branchiae ; 

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



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 or entirely wanting. They swim 

 by means of a flattened ventral fin, or by an elongated 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 ven- 

 tral fin being a modified " propodium." The "epipodia" are 

 apparently altogether wanting. Respiration is sometimes car- 

 ried 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 Firolidce 

 and Atlantid&i 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 operculum 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. 



