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. [Illustration: FIG. 162.--NUDIBRANCHS 1. _Dendronotus arborescens._ 2. _Tritonia plebeia._ 3. _Triopa claviger._ 4. _Ægirus punctilucens_] In fig. 162 we represent four species. Two of these--_Triopa claviger_ and _Ægirus punctilucens_--belong to the family _Doridæ_, the members of which are popularly known as Sea Lemons, and are distinguished by the presence of plume-like gills situated on the middle of the back. Another family (_Tritoniadæ_), characterised by the arrangement of the gills along the sides of the back, and by tentacles that can be retracted into sheaths, is represented by _Tritonia plebeia_ and _Dendronotus arborescens_ in the same figure, and by _Doto coronata_ in fig. 161. The family _Æolidæ_ also have their gills arranged along the sides of the back, but they differ from the last in that their tentacles are not retractile. They include the two species numbered 3 and 4 on fig. 161. The remaining one on fig. 161--_Elysia viridis_--is a member of the family _Phillirhoidæ_, characterised by a pair of tentacles on the dorsal side of the head and by the foot being either very narrow or absent, the latter feature denoting that the animals are not adapted for creeping on the bottom. In fact, several of the species of this family swim freely by means of flattened tails. The Tectibranchs are similar in general structure, but are very different in appearance, inasmuch as the gills, so prominent in the last division, are here covered by the mantle, or by the shell, which is often well developed. The latter is very variable in form, being of a globular, twisted, spiral, or other shape, but is sometimes absent in the adult. In fig. 163 we give a few examples of the shells of British species; and one (_Bulla hydatis_) is shown on Plate V. [Illustration: FIG. 163.--SHELLS OF TECTIBRANCHS]