Shell-less Pteropod (_Clione limacina_). _a._ Dorsal view. _b._ Ventral aspect. ] About a hundred species are known. [Sidenote: Case 96.] The Sea-Hares, so called on account of a slight resemblance to a crouching hare and not for their nimbleness of foot, are found in most parts of the world, in pools at low water. At the hinder part of the back two flaps of the mantle partly conceal a thin horny shell which serves as a protection to the gills and vital organs beneath. When molested, these animals discharge a large quantity of a purple fluid, discolouring the surrounding water for a distance of more than a yard. [Illustration: Fig. 18. Sea-Hare (_Tethys (Aplysia) punctata_). British. _a_, labial tentacles; _b_, upper tentacles or rhinophores; _c_, siphonal fold of the mantle near the shell; _d_, eye. ] [Sidenote: Case 97.] The shell of _Umbraculum_ is shaped very like that useful article, an umbrella, of the Chinese pattern. The animal is very large, having its breathing-organs on the right side below the shell. [Sidenote: Case 97.] The _Nudibranchs_ or Naked-gilled Molluscs comprise some of the most beautiful and strange forms. They are unprovided with shells except in the earliest stages of their existence, when they dwell in a minute nautiloid shell, furnished with an operculum, both of which are subsequently cast off. Unfortunately the colours of these beautiful creatures cannot be preserved after death, and therefore a small series of glass models is exhibited, which will give some idea of their great variety in form and colouring. They are found in most parts of the world, chiefly in shallow water, but a few species live upon floating seaweed in the open sea. Over a hundred species exist on the British coast, the majority of which are, however, very small. They are chiefly carnivorous, feeding on other molluscs, sea-anemones, &c. [Illustration: Fig. 19. The Umbrella-shell (_Umbraculum mediterraneum_). _a_, shell; _b_, gills; _c_, tentacles; _d_, mouth; _e_, foot. ] [Illustration: Fig. 20. Naked-gilled Mollusc, or Nudibranch (_Doto coronata_). _a_, head; _b_, foot; _c_, gills; _d_, tentacle-sheath; _e_, tentacle. ] Order 2.—PULMONATA. [Sidenote: Cases 97–135.]