540 LECTURE XXII. 



Class Gastropoda. 



The transition from the Pteropoda to the present class appears to 

 be made by the Philliroe. the Glaucus, the Carinaria, and the Firola^ 

 all floating pelagic genera remarkable for the delicacy of their tissues, 

 and the rudimental character of their gastric foot. These aberrant 

 Gastropods manifest the same affinity to the preceding group by the 

 presence, in some, of lateral, symmetrical, more or less aliform ex- 

 pansions, and, in others, of a shell characterised by its elegant sym- 

 metry, lightness, and transparency ; that of the Carinaria much re- 

 sembles the shell of the Cymhulia in form, but is of a calcareous 

 texture. 



The typical Gastropods, 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 animals, many of which appear to have superseded the extinct in- 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 mo- 

 difications, of which systematic naturalists have availed themselves 

 in distributing the numerous and diversified members of the class 

 into orders. 



In certain small shell-less marine genera, e. g. Rhodope, Tergipes, 

 Eolidina, no distinct respiratory organs have been detected ; these 

 form the order Apneusta. 



Where those organs are present they are, in the lower forms of the 

 class, exposed. The genera which support them on the back, or the 

 sides of the back, as the Glaucus^ Scyllcp.a, Tritonia, form the order 

 Nudihranchiata, 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 when 

 mature. The genera in which the gills have a similar position, but 

 extend around the body, as in 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 



