THE ORIGIN OF THE MARINE FAUNA. 177 



In the first place we find, when we take 

 a general survey of the animals that live in the 

 surface water, that they are all specially modified 

 in some way or another in structure or develop- 

 ment in adaptation to the peculiar conditions of 

 their life. The long spines of the Foraminifers 

 and the Crustacean larvae, the air bladders of 

 the Portuguese men-of-war, the oil drops of 

 the Copepods, the raft of the Mollusc Janthina 

 are, as we have seen, among the characters which 

 distinguish this peculiar Fauna. Now, when we 

 compare these surface-dwelling forms with their 

 nearest relations in the shallow waters, the con- 

 clusion we come to is that these features have 

 been acquired by the ancestors of the former, 

 which may have been similar in some respects to 

 those now living in shallow water. 



Some of the Gastropods of the shore-waters 

 have a simple cup-shaped shell like that of the 

 common Limpet, but the great majority of them 

 have a shell that is twisted up into a spiral 

 form. This twisting of the shell is, of course, 

 due to the twisting of the mantle or fold of skin 

 which secretes the calcium carbonate of which 

 the shell is mainly composed; and, when we 

 study the internal anatomy of the animal we 

 find that the shape of the mantle is associated 

 with a loss of the organs of one side of the body. 

 To put a long story into a few words, we may 

 say that the Gastropods with twisted shells are 

 lop-sided. Now when we examine the shells of 

 the Gastropods that live in the surface waters of 

 the ocean we notice that their shells are (with 

 a few exceptions such as Janthina) perfectly 

 M 



