CHAP. VI. HELICID.E, OB SNAILS. l6l 



distinctly than in any others, pointed out the mode 

 which she pursues in the development of all her 

 groups. Commencing from the most simple form, she 

 gives us the first germ of the limaciform structure in 

 the extraordinary genus Herpa, hereafter noticed. From 

 this point, as it were, she makes two diverging series ; 

 the one to the right hand, through the terrestrial slugs ; 

 the other to the left, through the semi-aquatic slugs 

 and the freshwater snails : and these two series, after 

 passing through numerous links, finally meet in the 

 pupaceous- formed shells, constituting the genera of 

 Pupa and Clausilia. It is hy this theory we account 

 for the actual presence of naked mollusks among the 

 Helicidts; and in this manner do we consider that the 

 whole form a circular group, the particulars of which 

 will be more clearly stated hereafter. 



(152.) The HELICID^E are the only animals of the 

 Testacea which breathe atmospheric air. From this 

 circumstance they have been considered, by some, in 

 the light of a distinct order ; but their close and inti- 

 mate connection with the Turbida on one hand, and 

 even with the Trochidce on the other, clearly shows 

 their situation to be intermediate, and that they, there- 

 fore, only constitute a family group. The respiratory 

 organs of these animals simply consist of a moderate- 

 sized oval aperture, placed under the reflected edge of 

 the mantle, which can be dilated or contracted at plea- 

 sure ; hence there are no branchia, but merely, ac- 

 cording to Cuvier, " a network of pulmonary vessels, 

 which spread over the parietes and pulmonary cavity." 

 The number of tentacula is variable, according to the 

 different groups ; but they are usually four, two long 

 and two very short ; the former bear the eyes at their 

 extremities, but these organs are only so far developed 

 as to appear like black points. So far as we yet know, 

 the greatest uniformity pervades the animals of all the 

 Helicince, or turbinated land-snails, so that the minor 

 divisions rest entirely on the structure of their shells ; 

 but in the spiral snails (or the sub-family Achatince) 



