They are usually divided into two sections, those with exposed or naked gills (Nudibranchiata) forming the first, and those in which the gills are covered either by the shell or the mantle (Tectibranchiata) comprising the second.
In the Nudibranchs the shell exists only during the embryonic stage, and the external gills are arranged on the back or along the sides. The tentacles are not employed as organs of touch, but are probably connected only with the sensation of smell, being provided with filaments of the olfactory nerve; the eyes are small dark-coloured spots embedded in the skin behind the tentacles. Various species are to be found on all rocky coasts, where they range from low-water mark to a depth of fifty or sixty fathoms, but a few are pelagic, living on the surface of floating sea weeds.
It is almost impossible to identify the species of nudibranchs from dead specimens, for the classification of the section is based largely on the arrangement of the gills, which are almost always retracted in the dead animals. This is also the case even with living specimens when disturbed or removed from the water; hence they should always be examined alive in sea water, while the animals are extended and moving.
Fig. 161.—Nudibranchs
1. Doto coronata. 2. Elysia viridis. 3. Proctonotus mucroniferus. 4. Embletonia pulchra
It will be understood from the above statements that special methods will be necessary when it is required to preserve specimens for future study, the gills being always retracted when the animal is killed for this purpose by any rapid process. We have found two methods, however, that are fairly satisfactory in the majority of instances.—Place the living animals in a suitable vessel of sea water, and leave them quite undisturbed till they are fully extended, and then either gradually raise the temperature till they are dead, or introduce into the water, cautiously, a solution of corrosive sublimate. In the latter case a much larger proportion of the sublimate will be required than when used for a similar purpose with freshwater molluscs. When the animals are dead it will be found that their gills are more or less extended, sometimes fully so, and they may then be transferred to diluted spirit or a two per cent. solution of formaldehyde.
