309] PROTEOCEPHALIDAE—LA RUE 309 



to breed and hence each individual spends part of its life in a marine 

 environment. The Gadidae are characteristic salt water fish altho the 

 members of one genus, Lota, have permanently established themselves 

 in the freshwater environment. At the present time there is no evi- 

 dence to show that species of Salmonidae or of Anguilla at the time 

 of their migrations introduce into freshwater parasitic species which are 

 capable of establishing and maintaining themselves in the new environ- 

 ment. Ward (1910) has pointed out that Salmo sehago, landlocked 

 in lakes of eastern North America, has a parasitic fauna that is charac- 

 teristic of its freshwater environment. It is evident then that what- 

 ever parasites this species brought with it from its former marine 

 environment were unable to establisli themselves. This failure may 

 have been due to a lack of proper intermediate hosts. 



These considerations lead to a possible explanation which depends 

 on the possibility that the intermediate host species (a fish or perhaps 

 an invertebrate) of a Tetraphyllidean acquired the habit of living in 

 brackish and finally in fresh water. The fish species which normally 

 made this intermediate host its food may also have made this change 

 of environment or perhaps other fish began to prey on the intermediate 

 host. Under these conditions the parasite might have become established 

 in the new environment. It also seems probable that new intermediate 

 hosts might have been acquired. The acquiring of a new host depends 

 not on the anatomical structures of the host but upon its physiological 

 properties and upon its habits of feeding and its environment. The 

 fact that the head and the suckers of Proteocephalus singularis strongly 

 resemble these structures in certain species of other families of Tet- 

 raphyllidea is suggestive. The host, Lepisosteus platostomus, is more- 

 over a primitive fish of strong carnivorous habits. The Lepisosteidae 

 have long been established in fresh water. It is here suggested that 

 some member of this family may have been responsible for the intro- 

 duction of Proteocephalids into the fresh water environment. The Pro- 

 teocephalids of amphibians and aquatic reptiles may have been acquired 

 in the course of development from some fish-like ancestor or it is more 

 probable that they have acquired their Proteocephalid fauna from eating 

 intermediate hosts (invertebrates or fish) infected with larval Proteo- 

 cephalids. In this connection it is of interest to note that the head of 

 Crepidobothrium, found in the Boidae, is the most primitive of all the 

 heads of Proteocephalids of amphibians and reptiles. That is to say, 

 it is more like the heads of other Tetraphyllideans than are the heads 

 of its congeners. The Boidae are considered to be somewhat primitive 

 snakes. 



