DISTRIBUTION OF ORGANISMS. 159 



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 & class of many families and genera, 

 including limpets, whelks, cowries, snails, etc. have compa- 

 ratively a high organization, the nervous system more con- 

 centrated, the nutritive more elaborate, but yet are of slug- 

 gish habits, usually moving by alternate contractions and ex- 

 pansions of a fleshy disk placed upon their stomachs ; hence 

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

 others possessed of but slender protection. A large propor- 

 tion 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 Gas- 

 teropoda as a 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. The descent of the class appears to be 

 from some families of the preceding ; for " they all," says a 

 minute observer of nature, ( 74 ) " commence life under the same 

 form, both of shell and animal ; namely, a very simple spiral, 

 helicoid shell, and an animal furnished with two ciliated 

 wings or lobes, by w r hich it can swim freely through the fluid 

 in which it is contained. At this stage of the animal's exist- 

 ence, it corresponds to the permanent state of a Pteropod." 



In the univalve mollusks, as in the bivalves, it clearly 

 appears that the humblest families are destined to a fixed 

 place in the depths of the ocean. As we advance through the 

 higher groups, we find, in parallel steps with an improvement 

 in the organs of animal life (for example, the splitting of the 

 sexes into different individuals), an advance in the sphere of 

 existence, to a life on the surface of the ocean, to fresh water, 

 and even to dry land. The humble Helicidce (snails) a 

 family of the Gasteropoda, are the first animals which we en- 

 counter as adventuring upon the firm surface of the globe. 

 And it is interesting to remark, in this progression, the 



