Fam. 3. Cancellariidae.—Proboscis short, usually no radula, shell oval, columella strongly plicate; no operculum. Cretaceous——. Single genus, Cancellaria (subg., Merica, Trigonostoma, Admete).
CHAPTER XV
CLASS GASTEROPODA (continued): OPISTHOBRANCHIATA AND PULMONATA
Order III. Opisthobranchiata
Visceral loop not twisted (except in Actaeon) in a figure of 8 (Euthyneurous type, p. [203]), auricle usually behind the ventricle, ctenidium often replaced by secondary branchiae, pallial cavity, if existing, more or less open, shell present or absent, operculum absent (except in Actaeon), animal hermaphrodite, with separate sexual openings, marine only.—Carboniferous to present time.
The character of their nervous system decisively removes the Opisthobranchiata from the Prosobranchiata, and approximates them to the Pulmonata. Actaeon, however, which is streptoneurous, as well as possessing an operculate shell with prominent spire, forms an interesting link with the Prosobranchiata. At the opposite extreme to Actaeon stand forms like Siphonaria and Gadinia, which are probably close links with the Pulmonata (p. 19). The generative system of the whole group, which is, as in the Basommatophora, of the hermaphrodite type, without mutual fecundation, is another link of connexion with the Pulmonata. The respiratory organs present the most varied forms, sometimes consisting of one ctenidium (never two), sometimes of secondary branchiae, variously placed, while sometimes no special organ exists.
The prolongation of the foot into lateral epipodia or parapodia (possibly to aid in swimming), and the effect of the epipodia upon the shell, according as they involve it completely or partially, are among the most instructive features of the Opisthobranchiata. If the epipodia are developed on the anterior portion of the body, and do not become reflected, they may, as in most Pteropoda Thecosomata, not directly affect the shell. But when, as in the Tectibranchiata, the epipodia are medio-lateral, and tend to envelope the shell, their effect may be traced by a series of forms varying in proportion to the amount of shell-surface covered by the epipodia. The two principal lines along which modification takes place are the gradual reduction of the spiral nature of the shell, and the gradual lessening of its solidity. Both these changes are the direct result of the additional protection afforded to the visceral mass by the reflected epipodia, which renders the existence of a shell less and less necessary. A precisely similar line of change is seen in the Pulmonata, culminating in forms like Arion (p. [174]).
Fig. 286.—Illustrating the transition of form in the shell of Tectibranchiata from the pointed spiral to the almost flattened plate: A, Actaeon; B, Aplustrum; C, Cylichna; D, Atys; E, Philine; F, Dolabella; G, Aplysia; H, Pleurobranchus. (Not drawn to scale.)
Fig. 287.—Illustrating the gradual covering of the shell in the Tectibranchiata by the epipodia and mantle: A, Haminea; B, Scaphander; C, Aplustrum; D, Aplysia; E, Philine; c.d, cephalic disc; ep, ep, epipodia; sh, shell. (Not drawn to scale.)