294 LECTURE XXII. 



The typical Gasteropods, characterised by the greater development, 

 especially the breadth of their ventral muscular locomotive disc, con 

 stitute a very extensive, and widely distributed class of Molluscous 

 animals, many of which appear to have superseded the extinct in- 

 habitants of the chambered shells in the organic economy of the ex- 

 isting shores. Most of the species are marine ; many inhabit fresh 

 waters; a few are terrestrial. They offer corresponding conditions of 

 the respiratory organs in relation to these media, with minor modifi- 

 cations, of which systematic naturalists, and especially Cuvier, have 

 availed themselves, in distributing the numerous and diversified mem- 

 bers of the class into orders. 



In the lower organised Gasteropods the respiratory organs are ex- 

 posed ; those genera which support them on the back, or the sides of 

 the back, as the Glaucus, Eolida, Tritonia, form the order Nudi- 

 branchiata, in which all the species are without shells, in the mature 

 state; those genera which carry the gills at the lower part of the 

 sides of the body, between the foot and mantle, as the Phyllidia, 

 constitute the order Inferohranchiata : they are likewise naked or 

 shell-less. The genera in which the gills have a similar position, but 

 extend around the body, as the Patella and Chiton, form the order 

 Cyclobranchiata : they are defended by a conical shell composed of 

 one or many pieces. 



In the rest of the class the respiratory organs are concealed. Those 

 genera, as the Aplysia and Bulla, which have the gills protected by 

 a fold of the mantle containing a rudimental shell, or by a reflected 

 process of the foot, form the order Tectibranchiata. Those genera, 

 as Limax or Helix, which have a vascular air-sac or lung, protected 

 by a rudimental, or fully developed shell, form the order Pulmonata. 

 A small order of marine Gasteropods, including the Fissurella and 

 Haliotis, which have their pectinated branchiae protected by a wide 

 shield-shaped shell, is called Sciitibranchiata. Another small order, 

 in which similar branchiae are protected, with the entire body, by a 

 tubular shell, is called Tid>ulibranchiata. 



In all the foregoing oi'ders of Gasteropods the male and female 

 organs of generation are associated in the same individual ; in the 

 last and highest organised order, called Pectinibranchiata, the sexes 

 are distinct. 



The soft parts of the Gasteropods are immediately invested by a 

 soft inarticulated lubricous integument called the mantle, in which 

 may be recognised an epidermal, a pigmental, and a dermal layer, 

 the latter being highly contractile. The shell results from the meta- 

 morphosis and calcification of cells deposited in layers beneath the 

 epidei'mis, in the situation of the rete-mucosum in the human integu- 



