164 PRIMITIVE FORMS OF LIFE 



worms, and Leeches derived from them, contrasting with the 

 differentiation of the sexes so general in marine Annelid 

 Worms. 



Exactly the same thing happened in the case of Gasteropod 

 Pulmonate Molluscs, of which the Snail is the common type, 

 and which are represented by countless species in the fresh 

 waters and in all damp land areas. All are hermaphrodite, 

 whereas the marine Gasteropods, with their helicoidal shells, 

 and gills protected in a special cavity situated in front of the 

 dorsal cone, 1 are all unisexual. 



The marine Lamellibranchs, which lead a sedentary life, 

 are often hermaphrodite, like fixed organisms ; hermaphroditism 

 is also definitely protandrous in the Oysters, which are fixed 

 like the Tunicates. 2 We have very little information as to the 

 sexual conditions of the other Lamellibranchs. 



Objections might be raised against the theory that 

 hermaphroditism is due to a precarious source of food supply, 

 particularly in fresh waters and on land, namely the existence 

 of hermaphroditism in true parasites, such as the liver flukes 3 

 of Sheep and similar animals 4 and in the Turbellarians, which 

 are free-living and form together with them the order of 

 Flat Worms, from which, however, the Nemerteans are to be 

 excluded ; and it may also be contended that every order of 

 marine Gasteropod Molluscs without an anterior branchial 

 cavity consists of hermaphrodites. However, a very few 

 words will suffice to deprive these objections of all validity. 



In the first place, although the organization of the 

 hermaphrodite Flat Worms is so degraded that it has been 

 attempted on various occasions to describe them as primitive, 

 their double genital apparatus has preserved a complicated 

 structure of a very special, constant, and definite type ; this is 

 enough to show that we are here dealing with a group of 

 degenerate organisms, sprung from a higher group. The only 

 possible starting-point for this degeneration is the Leech, 

 whose class manifestly derives from the Earthworm, whose 

 organization is so similar that a man like Franz Vejdowsky, 5 



1 They are called Prosobranchatata. 



2 LVI. 



3 They form the two classes of Flat Worms, the Trematodes, and the 

 Cestodes, or Tapeworms. 



4 They are called Opisthobranchiata. 



5 LVII, 38. 



