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.[403]
The Opisthobranchiata may be classified as follows:—
| Opisthobranchiata | 1. Tectibranchiata | Bulloidea |
| Aplysioidea | ||
| Pleurobranchoidea | ||
| Siphonarioidea | ||
| 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.
Section I. Bulloidea.—Shell more or less spiral, internal or external, epipodia more or less developed, a broad cephalic disc, distinct from the dorsal region, usually no tentacles, eyes sessile.
Fam. 1. Actaeonidae.—Shell spiral, solid, entirely covering the animal; spire generally prominent, operculum corneous, visceral loop streptoneurous, no epipodia, radula multiseriate, teeth numerous, very small. Carboniferous ——. Genera: Actaeon (Fig. [286]a); Volvaria (Tertiary), Fortisia (Eocene) Actaeonina (Carboniferous), Cylindrites (Secondary strata), Actaeonella (Cretaceous).
Fam. 2. Tornatinidae.—Shell spiral, cylindrical, entirely covering the animal; spire concealed, cephalic disc with two large tentaculiform appendages behind, no radula. Genera: Tornatina (= Utriculus), Volvula.
Fam. 3. Scaphandridae.—Shell more or less external, covering all or nearly all the animal, spire concealed, cephalic disc simple or notched behind, epipodia well developed, radula with first lateral very large, stomach sometimes with powerful gizzard. Genera: Scaphander (Fig. [287] B); Sabatia (Pliocene), Smaragdinella, Atys (Fig. [286] D), Cylichna (Fig. [286] C), Amphisphyra.
Fam. 4. Bullidae.—Shell external or partly internal, spire quite or nearly hidden, cephalic disc broad, without appendages, epipodia often large; radula usually multiseriate. Genera: Bulla (subg. Haminea), Acera, mantle with long filiform appendage, epipodia touching over the shell; Cylindrobulla, Volvatella.