84 CHARACTEKS OF MOLLUSCA. [CH. II 



make the group their special study. The relationships 

 therefore of the different genera is in many cases obscure. 

 They can therefore not be made use of in a strictly 

 scientific comparison of the faunas of various parts of the 

 world. We know how varied may be the structure of 

 other animals — even belonging to the same group — under 

 a superficial similarity ; for example the genus Doriopsis 

 among the Nudibranchs differs from the genus Doris 

 (sensu lato) by the total absence of that most character- 

 istic molluscan organ the odontophore ; and yet with this 

 important dissimilarity it would be impossible to separate 

 the two genera by any marked external characters. 

 Were one totally unacquainted with their internal 

 structure the Brachiopods and Lamellibranchs might be, 

 as they have been in the past, associated closely together. 

 Analogous differences may separate some of the genera 

 of Pulmonata whose anatomy is not known. 



In the second place they are very capable in some 

 way or other of crossing the ocean, for we find them in 

 oceanic islands which cannot have been stocked inde- 

 pendently with them. Mr Wallace remarks that "they 

 have no means of passing over the sea but such as are 

 very rare and exceptional." He quotes however Darwin's 

 experiment to the effect that a Roman snail lived after 

 immersion in salt water for twenty days. For further 

 examples of the ways in which the terrestrial Mollusca 

 can cross the sea the reader is referred to a later 

 chapter. 



If it were not for these objections the land Mollusca 

 would be a most important group ; they are extremely 



