532 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 2 



in the same region nor in a remote region, but in a neighboring region, 

 separated from the first by a barrier of some sort, not freely traversible." 



In view of these significant comparisons of free-living animals of the 

 two oceans, a comparison of some of the parasites of these animals becomes 

 a subject of some interest. The geographical distribution of parasites has 

 often been found to furnish considerable light on problems of relationship 

 among hosts and to furnish evidence of earlier distribution of the hosts as 

 well as probable land connections of the past. Conclusions rely chiefly 

 upon a certain amount of host specificity on the part of parasites, a char- 

 acteristic almost always exhibited to a greater or lesser degree. 



Von Ihering (1902) proposed that genetic relationship among para- 

 sites was a good index of genetic relationship of the hosts involved and 

 that two host species were related if they contained identical or similar 

 parasites. Zschokke (1904) applied this principle to the distribution of 

 certain cestodes of marsupials. Perhaps the outstanding classical example 

 of this type of research is that of Metcalf ( 1923), who found that certain 

 ciliated protozoa of South American and Australian Leptodactylidae are 

 so similar that a genetic relationship between these amphibia of the two 

 continents is clearly indicated and, as a corollary, a one-time land con- 

 nection between the two continents. Similar studies have been made on 

 the distribution of bird lice (Kellogg, 1905). 



The author (Manter, 1940) recently reported 82 species of digenetic 

 trematodes from marine fishes of the tropical American Pacific, a region 

 hitherto unexplored for such parasites. These trematodes proved to be 

 generally different from those found in other parts of the world with the 

 exception of the tropical Atlantic. A rather large number of the species 

 (23, or more than 25%) are known to occur also in the American 

 Atlantic, and 13 species are actually (so far as is known) endemic amphi- 

 American species not known elsewhere. Furthermore, 11 additional 

 species from the Pacific find their nearest related species in the Atlantic. 

 At least 7 of these 1 1 species are so closely similar to their Atlantic rela- 

 tives that they could be termed geminate species. Only 7 species of the 83 

 are known to occur elsewhere other than in the Atlantic. Six of these 7 

 are reported from Japan. 



It should be noted that trematodes of the tropical Pacific are still 

 incompletely known. While all of the digenetic trematodes collected are 

 considered here, these probably represent not more than a good sampling 

 of the region. The same incomplete knowledge of the trematode fauna of 

 the more northern Pacific and Atlantic coasts of America exists. Marine 



