LAND AND WATER 17 



Peripatus, Lepisma, Gryllus, Forficula, and 

 others ; has rightly interpreted the direct com- 

 parability with the vertebrate trophoblast, and 

 has looked upon it (as I have done ['95 B] for 

 vertebrates) as an adaptation to a viviparous 

 habit acquired by the terrestrial descendant of 

 an aquatic ancestor." . 



An example of a simple and obvious physio- 

 logical system of classification, based upon dis- 

 tribution and covering the entire animal kingdom, 

 is that which distinguishes between aquatic and 

 land animals. There is a peculiar propriety in 

 this disposition inasmuch as all the groups of 

 land animals have had a more or less remote 

 aquatic ancestry, some inconceivably remote, 

 others comparatively recent ; whilst others again 

 have reverted to an aqueous medium in which 

 their lives are spent. Land mollusca, land 

 leeches, land nemertines, land planarians, and 

 land crabs, are examples of terrestrial groups 

 which have had a recent aquatic ancestry, their 

 immediate congeners being still aquatic animals. 



The general structural differences which are 

 associated with the radical change of environment 

 are chiefly connected with the organs of respira- 

 tion and, to a less extent, those of locomotion. 

 In some cases, as with the operculate land 

 molluscs, the change from water to land has 

 only affected the organ of respiration. In others 

 no striking alteration is manifested in essential 

 organs, although accessory structures sometimes 



B 



