We shall now consider the Gasteropoda, which is divided into four orders. Firstly, Nucleobranchiata, animals which float on the surface of the ocean: they are Diæcious, or in separate sexes, and the nervous system is widely distributed in the body, the shell, in Carinaria, for instance, covering only a very small portion of the body. The first family of this order is Atlantidæ, of which the types are the fossil Bellerophon and the recent Atlanta. The second family is Firolidæ, the types of which are Carinaria and Firola. Carinaria or glass nautilus is shaped like the bonnet-cap shell, Pileopsis. It is as transparent as glass; and although now very common, was formerly one of the most highly-prized shells by collectors. The second order of Gasteropoda is Opistho-Branchiata, and is divided into two sections, the Nudibranchiata, and the Tectibranchiata. The Nudibranchiata have no shell except in the larva state; they mostly live at the bottom of the sea on rocky shores, but a small number swim on the surface. They are remarkable for their variety of form and vivid colouring, being the most beautiful of all molluscous animals; they may truly be called the caterpillars of the sea, for their branchiæ remind us of the spines with which many lepidopterous larvæ are covered. The first family is Elysiadæ, types Limapontia and Elysia. The second is Phyllirhoidæ, type Phyllirhoe. The third is Æolidæ, types Glaucus and Æolis. The fourth is Tritoniadæ, types Scyllæa and Tritonia. The fifth is Doridæ, types Idalia and Doris, the curious sea-lemon. The first family of the second division, Tectibranchiata, is Phyllidiadæ, types Diphyllidia and Phyllidia. The second family is Pleurobranchidæ, types, Umbrella, in form resembling a limpet, and Pleurobranchus. The third family is Aplysiadæ, types, Dolabella and Aplysia. The fourth family is Bullidæ, types, Scaphander, Acera, and Bulla. The fifth family is Tornatellidæ, types, Tornatina and Tornatella. The third order is the Pulmonifera, and the fourth is the Prosobranchiata; we shall speak of them in the next chapter. In this family we reach a group of Gasteropods much more numerous, both in species and in special types, which respire by the aid of branchiæ, or gills. Cuvier divides them into many orders, based chiefly upon their respiratory organs. The Tectibranchiata have the gills attached either to the right side of the body or upon the back, arranged in the form of leaflets, more or less divided, but not symmetrical, and nearly covered by the mantle. _Bulla_ and _Aplysia_ are the two principal genera of the group, and may be considered as the type of two small families.