. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 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 " ; 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
. A manual of zoology for the use of students : with a general introduction on the principles of zoology . Zoology. 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 " ; 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 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
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