No. 624] SHORTER ARTICLES AND DISCUSSIOX 



91 



Limited geographical areas have been covered in the sur- 

 veys for cercarias. Two drainage systems of the Atlantic slope, 

 isolated regions around the Great Lakes, a portion of the upper 

 Columbia and an isolated region in Wyoming, together with 

 more widely investigated areas in the Mississippi basin, consti- 

 tute the localities in which collections have been made. The 

 entire south, southeast and southwest constitute vast unexplored 

 areas, the former two of which should yield a great number of 

 species. In addition, the variation of species of flukes in snails 

 from one season to another makes it highly probable that many 

 more species occur in the Mollusca of the areas surveyed than 

 the records show. Table I shows that one distome species, Cer- 

 caria megalura, has been found in Gmiohasis virginica from the 

 Atlantic slope, and in Pleurocerca elevatum from the Mississippi 

 basin ; and that C. inhahilis and C. diastropha have been found 

 on both the eastern and western slopes of the Mississippi drain- 

 age. On the other hand, none of the species described for 

 the Bitter Boot Valley have been recorded east of the Rocky 

 Mountains. 



Records of percentage of infection from larval flukes vary 

 from a few hundredths of a per cent, for certain cercarise de- 

 scribed by Sisnitzin in 1911 from the Black Sea to a heavy infec- 

 tion of every individual of a particular species in a locality. 

 The lowest infection record for the United States is one per cent. 

 (C. fusiformis in Physa gyrina). On the other hand, several 

 heavy infections have been recorded, including three with total 

 infection. The mollusks most heavily infected are the ubiqui- 

 tous species, Planorhis trivolvis and Physa gyrina, and the 

 western species, Lymnaa proxima. In the case of the Planorhis 

 and the Lymnaa the average heavy infection is caused by dis- 

 tome cercarisB. The heavy infection among the physas is caused 

 by monostome and holostome larva?. 



Table II, which summarizes the infection from the host point 

 of view, shows that Lymnaa proxima has the greatest number 

 of species per habitat. Planorhis trivolvis has been found to be 

 infected in the greatest number of localities, while Physa gyrina 

 is the only mollusk to harbor three groups of Digenea. Of the 

 sixty named species listed in Tables I and II eleven are recorded 

 from two hosts. 



Accompanying the cercariae in the mollusks are the parthenitai 

 (sporocysts and redise) of these cercarise. The cercariae develop 

 parthenogenetieally within these parthenitae. Typically, as in 



