3l6 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 fiinction is performed simply by the walls of 

 the palHal chamber. 



The'Ifeterepodd are divided into the two families FiroUda 

 and Atlantidce, 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 Gast^ofdda 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. 113. — Lintax Sowerfyi, one of th« 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. 113) have a rudiment- 

 ary shell, which is completely concealed within the mantle. 

 Others are entirely destitute of a shell. They arg 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 



