CHAPTER XV. Class Gasteropoda (_continued_): Opisthobranchiata and

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).

The habits of life of the Opisthobranchiata are very varied. Some, especially the heavier types, burrow in sand, and are then usually furnished with a broad cephalic disc, as a digging apparatus; some (certain Bulla) flit about in shallow pools on mud flats; others (Phyllirrhoe and the Pteropoda) swim freely in the open sea; others (most Nudibranchiata) crawl slug-like on sea-weeds or corallines, and in colour singularly harmonise with their environment (p. 71 f.); others again (Siphonaria, Gadinia), stick limpet-like to rocks between tide marks. As a rule, they occur only in clean salt water, but Embletonia has been found in the Victoria Docks at Rotherhithe, as well as in parts of the Baltic, where the water has only 7 parts of salt in 1000, while Limapontia occurs in nearly fresh water at Bornholm and Gothland.

Their food varies greatly. As a rule, they are frugivorous, but many cases of carnivorous habit occur. Scaphander has been seen to swallow Dentalium six at a time, and in six hours the shells of all were reduced to tiny fragments. Glaucus devours the soft portions of the pelagic Porpita and Velella; Idalia elegans eats its way into the test of Ascidians, and completely buries itself in the body of its prey.

The Opisthobranchiata may be classified as follows:--

{ Bulloidea { 1. TECTIBRANCHIATA { Aplysioidea { { Pleurobranchoidea { { Siphonarioidea { =Opisthobranchiata= { 2. ASCOGLOSSA { { 3. NUDIBRANCHIATA { Cladohepatica { { Holohepatica { { 4. PTEROPODA { Thecosomata { { Gymnosomata

=Sub-order I.= =Tectibranchiata.=--Right ctenidium usually present, more or less concealed by the mantle fold, visceral ganglia united by a very long commissure, shell variable in form, more or less enveloped in folds of the mantle and foot, often becoming rudimentary.