338 MANUAL OF ZOOLOGY. 



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 Firolida 

 and Atlantidcz, 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 neural, and the sexes are united 

 in the same individual. 



Fig. 141. Limax Soiuerbyi, one of the Slugs. (After Woodward.) 



The Pulmonifera include the ordinary land-snails, slugs, 

 pond-snails, &c., and are usually provided with a well-developed 

 shell, though this may be rudimentary (as in the slugs), or. 

 even wanting. Though formed to breathe air directly, many 

 of the members of this sub-class are capable of inhabiting 

 fresh water. The common Pond-snails are good examples' of 

 these last. The condition of the shell varies greatly. Some, 

 such as the common Land-snails, have a well-developed shell, 

 within which the animal can withdraw itself completely. 

 Others, such as the common Slugs (fig. 141) have a rudimen- 

 tary shell, which is completely concealed within the mantle. 

 Others are entirely destitute of a shell. They are divided into 

 two sections as follows : 



Section I. Inoperculata. Animal not provided with an oper- 

 culum to close the shell. In this section are included the families 



