This order is defined as follows: "Shell rudimentary, or wanting. Branchiae arborescent or fasiculated, not contained in a special cavity, but more or less completely exposed on the back and sides, towards the rear (opisthen) of the body. Sexes united " (Milne-Edwards). (See Woodward's 'Manual.')

The Opisthobranchiata, or "Sea-slugs," may be divided into two sections, the Tectibranchiata and Nudibranchiata, according as the branchiae are protected or are uncovered.

The first section, that of the Tedibranchiata, is distinguished by the fact that the animal is usually provided with a shell, both in the larval and adult state, and that the branchiae are protected by the shell or by the mantle. Under this section are included the families of the Tornatellidae, Bullidae (Bubble-shells), Aplysiadae (Sea-hares), Pleurobranchidae, and Phylli-diadae.

In the second section, that of the Nudibranchiata (fig. 219), the animal is destitute of a shell, except in the embryo condition, and the branchiae (rarely absent, as in Limapontia and Rhodope) are always placed externally on the back or sides of the body. This section comprises the families Doridae (Sea-lemons), Tritoniadae, AEolidae, Phyllirhoidae, and Elysiadae.

Fig. 217.   Scalaria Graen landica, a Holostoma tous Univalve.

Fig. 217. - Scalaria Graen-landica, a Holostoma-tous Univalve.

Fig. 218.   Fusus tornatus, a Si phonostomatous shell. Post Pliocene.

Fig. 218. - Fusus tornatus, a Si-phonostomatous shell. Post-Pliocene.

Specimens of the Sea-slugs and Sea-lemons may at any time be found creeping about on sea-weeds, or attached to the under surface of stones at low water. The head is furnished with tentacles, which appear to be rather connected with the sense of smell than to be used as tactile organs; and behind the tentacles are generally two eyes. The nervous system is extremely well developed, and would lead to the belief that the Nudibranchs are amongst the highest of the Gasteropoda. Locomotion is effected, as in the true Slugs, by creeping about on the flattened foot.