MOLLUSCA PEOPER. 



205 



organs, these are in tlie form of gills, placed, without any protection, 

 upon the back or sides of the body. The head is furnished with 

 tentacles, which do not appear to be used as organs of touch, but are 

 more probably connected with the sense of smell ; and behind the 

 tentacles are generally two eyes. Locomotion is effected, as in the 

 true Slugs, by creeping about on the flattened foot. 



The last remaining group of the " branchiate " Gasteropods is that 

 of the Heteroixida (fig. 145), comprising a number of curious forms 

 which are found swimming at the surface of the open sea, instead of 

 creeping about at the bottom of the sea. In order to adapt them for 

 this mode of life, the foot, instead of forming a ci'eeping disc, is 

 modified to form a compressed fin (/). The Heteropoda are to be 

 regarded as the most highly organised of all the Gasteropoda, at the 

 same time that they are not the most typical members of the class. 

 Some of them can retire completely within their shells, but others 



Fig. 14.j. — Hcternpodrt. , Ci'rinaria cilmhium. p rinlm.scis ami mitntli; 

 t tentacle.s ; h Gills ; s Shell ; /Foot ; d Disc. (After W.ioJward.) 



have large bodies, and the shell is either small or entirely absent. 

 In Carinaria, which may be taken as a good example of the group, 

 there is a little limpet-shaped shell protecting the gills (i) and heart. 

 The animal swims, back downwards, by means of a vertically-flat- 

 tened ventral fin (/), on one side of which is a little sucking-disc 

 (r/), b}' which the animal can adhere at pleasure to floating sea- weed. 

 Carinaria is found in the Mediterranean and other warm seas, and 

 is so transparent that the course of the intestine can be see a along 

 its whole length. 



The last group of the class is that of the "air-breathing" Gaster- 

 opods, so well-known as Land-snails, Pond-snails, and Sings (fig. 

 146). All the members of this group are formed to breathe air 

 directly, instead of through the mediixm of water, and they therefore 

 never possess gills or bi-anchia;. In place of these they have a pul- 

 monary chamber or lung, formed by a folding of the mantle, and 

 having air admitted to it T)y a round hole on the right side of the 



