288 MANUAL OF THE MOLLUSCA. 



tached to the parietes of the * vitellary ' vesicle, whicli Gegenbaur 

 and Scliinidt compare to a "Wolffian body. 



" Gegenbaur draws attention to the fact, that the first rudi- 

 ment of the shell in Limax, Clausilia, and probably Helix, is not 

 secreted on the exterior of the mantle, as in other gasteropoda; 

 but is deposited, in the form of calcareous granules, within its 

 substance. 



*' Besides, therefore, the possession of "Wolffian bodies, and of 

 especial contractile organs, which subserve respiration and cir- 

 culation during embryonic life — the terrestrial gasteropoda are 

 further distinguished by the peculiar mode of development of 

 their shells — if the observations upon Clausilia and Helix may 

 be extended to the rest. The first development of the shell 

 within the substance of the mantle (a relation found hitherto 

 only in the Cephalopoda) is up to the present time a solitary 

 fact, without parallel among the other gasteropodous families." 

 (Huxley.) 



Family I. — Helicid^.* Land-snails. 



Shell external, usually well developed, and capable of con- 

 taining the entire animal ; aperture closed by an epiphragm 

 during hybernation, f 



Animal with a short retractile head, with four cylindrical, 

 retractile tentacles, the upper pair longest and bearing eye- 

 specks at their summits. Body spiral, distinct from the foot ; 

 respiratory orifice on the right side, beneath the margin of the 

 shell ; reproductive orifice near the base of the right ocular 

 tentacle ; mouth armed with a horny, dentated, crescent-shaped 

 upper mandible ; lingual membrane oblong, central teeth in- 

 conspicuous, laterals numerous, similar. 



Helix, L.| 



Type, H. pomatia, L., Eoman snail. 



Etymology, Helix, a coil. 



Shell umbilicated, perforated or imperforate ; discoidal, 

 globosely-depressed or conoidal ; aperture transverse, oblique, 

 lunar, or roundish ; margins distinct, remote, or united by 

 callus. 



Animal with a long foot, pointed behind ; lingual teeth usually 

 in straight rows, edge-teeth dentated. 



* The account of this family is chiefly taken from Dr. L. Pfeiffer's MonograpkUl 

 Heliceorum. 



t The epiphragm is a layer of hardened mucus, sometimes strengthened with car, 

 bonate of lime ; it is always minutely ppxfora' ed opposite the respiratory orifice. 



X The synonomy of the genus would fill several pages. See p. 48, 



