GASTEROPODS — THEIR ORDERS. 105 



organs, and Cuvier has ha])])ily availed himself of this to 

 divide the class into the following orders : — 



1. PuLMONES. — These breathe the atmosphere admitted 

 into a pulmonary cavity by a lateral orifice, which the snail 

 opens and shuts at pleasure ; some of them have no shell, 

 but the greater number are protected by one, which is often 

 completely turbinate ; the aperture, however, has never an 

 operculum. They form two large families, the terrestrial 

 (Pulmones terrestres), characterised by having almost all of 

 them four tentacula ; and the acjuatic (P. acjuatiques), which 

 have only two tentacula. Of the former, the slug (Limax), 

 and the snail (Helix), are the principal genera ; the latter 

 live in fresh waters, but incapable of extracting air from the 

 medium in which they live, they are necessitated to rise fre- 

 quently to the surface to obtain this. Our ditches contain 

 them in profusion, where you may readily observe their 

 motions, viz. the Planorbes, known by the shell formed, 

 like the fossil Ammonites, of a tube wound up sjjirally on the 

 same plane, bvit the cavity is, with a single exception, 

 simple and continuous ; while the shell of the Limneus is 

 turbinated like the periwinkles. 



2. NuDiBRANCHES. — These have no shell, and carry their 

 branchias of various forms on some part of the back exposed 

 to the direct influ- 

 ence of the circum- F'ff- ^■5. 



fluent waters (Fig. 

 15). They are na- 

 tives of the sea, 

 and, from their fi- 

 gure, may be called 

 sea-slugs. Cuvier 

 has not separated 

 them into families, 

 but the principal genera are admirably described, for they 

 were favourite objects of his stvidy. The Doris has its 

 branchiae, sometimes feathered like an ostrich-plume, placed 

 near the posterior extremity, and the creature has the power 

 of concealing them when in danger. The Tritonia is more 

 slug-like than the Doris, but its branchia; form a curled 

 fringe, interrupted at intei'vals, along each side of the back. 

 The Thethys has, besides, a largely expanded veil over the 

 mouth, imparting a very peculiar character to the genus. In 

 the Glaucus the branchia? are moulded into fingered fan-like 

 fins; while in Eolides and Tergipes they form conical or cy- 

 lindrical papillcTD, disposed in series along the back and sides. 



3. Inferobranches are similar to the preceding, but 



