180 



DISTRIBUTION OF ORGANISMS. 



of the whale. Professor Edward Forbes expresses his opinion 

 that the larva of the pteropod will yet most likely be found to 

 resemble an ascidian polype ; inferring a very brief descent 

 from the starting-point of life in its class. 



The Gasteropoda a class of many families and genera, in- 

 cluding limpets, whelks, cowries, snails, &c. have compara- 

 tively a high organization, the nervous system being more con- 

 centrated, the nutritive more elaborate ; but yet are of sluggish 

 habits, usually moving by alternate contractions and expan- 

 sions of a fleshy disk placed upon their stomachs, whence the 



FIG. 95. 



Eolis Inca, a naked Gasteropod. 



name of the class. Many of the gasteropods are naked, others 

 possessed of but slender protection. A large proportion are 

 vegetable feeders, the marine species battening upon sea-weed, 

 the terrestrial species upon herbage and fruit ; the rest are 

 flesh-eaters, but the general character of the Gasteropoda as a 



FIG. 96. 



' Enibryoes of Nudibrancldate Gasteropods. 



class is harmless, like that of the herbivorous mammalia. A 

 clear gradation of forms passes through some of the families, 

 from the simple cone of the limpet to the spiral of the snail 

 (Fig. 97). The descent of the class appears to be from some 



