This order is defined by the following characteristics: Animal provided with a shell, or not, free-swimming and pelagic ; locomotion effected by a fin-like tail or by a fan-shaped, vertically-flattened, ventral fin.
Fig. 219. - Nudibranchiata. Doris Fohnstoni, one of the Sea-lemons.
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; t Tentacles ; bBranchiae; s Shell; f Foot; d Disc. (After Woodward.)
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 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 Firolidae and Atlatitidae, 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.
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. 221. - Limax Sowerbyi, one of the Slugs. (After Woodward.)
The Pulmonifera include the ordinary Land-snails, Slugs,Pond-snails, etc, 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. 221) have a rudimentary shell, which is completely concealed within the mantle. Others are entirely destitute of a shell. They are divided into two sections as follows:
Animal not provided with an operculum to close the shell. In this section are included the families Helicidae, (Land-snails), Limacidae (Slugs), Oncidiadae, Limnaeidae (Pond-snails), and Auriculidae.
Shell closed by an operculum. In this section are included the families Cyclostomidae and Aciculidae.