NO. 16 MANTER: DISTRIBUTION OF DIGENETIC TREMATODES 545 



from southern South America, it is known that fish trematodes of colder 

 oceans differ greatly from trematodes of warm waters, and not only is 

 the Panama Canal too recent to explain the widespread similarity but 

 little transfer could take place through its fresh waters. Transfer by birds 

 is more possible. Only very rarely, however, would trematodes of fishes 

 survive long in a bird, so that such transfer would probably involve 

 passage from the bird of viable eggs of digested trematodes. Such a thing 

 is very possible but hardly seems sufficient to explain the widespread simi- 

 larity in many diverse genera of trematodes. Much more likely, it seems 

 to the writer, is the influence of a former continuity between the two 

 oceans known to have existed in the past. Persistence of this influence 

 might be expected to be greater in a group of endoparasites. Such an 

 interpretation is in harmony with observations made in other groups of 

 animals. 



In other words, conclusions from a study of digenetic trematodes 

 reiterate a claim that the Galapagos Islands represent an outpost of the 

 West Indies rather than an outpost of South America. A land connection 

 is not, of course, necessarily indicated. A relatively shallow water con- 

 tinuity between the two oceans is indicated. 



SUMMARY 



The digenetic trematode fauna of marine fishes of the tropical Ameri- 

 can Pacific shows a very marked similarity to that of the tropical 

 American Atlantic especially as compared with such trematodes in other 

 regions. 



This similarity extends to specific identity in the case of 23 of 82 

 species collected. Thirty-five of the 82 species (about 41%) are either 

 identical with species found in the American Atlantic or are more closely 

 related to Atlantic species than to species in other regions of the world. 



Fourteen species (about 17% of the total) are endemic amphi- 

 American species. These are practically always in related hosts, but in no 

 case were the hosts the same in the two oceans. 



Five genera (of 51) are endemic amphi-American genera (about 10% 

 of the total number of genera). 



Trematodes (39 species) from the Galapagos Islands show greatest 

 similarity to trematodes of Tortugas, Florida; some similarity to trema- 

 todes of the Pacific coast of Mexico, but relatively little similarity to 

 trematodes collected from the adjacent mainland of South America. 



