24 ILLINOIS BIOLOGICAL MONOGRAPHS [470 



mention of only three cercariae belonging to this group. The cercaria 

 of Diplodiscus subclavatus has been known for a long time, having been 

 first described by de Filippi and best described by Looss (1892 :162-166). 

 Of the two other forms one was first described by Sonsino (1892) as 

 Cercaria pigmentata, and later shown experimentally by Looss ( 1896 :- 

 185-191) to be the larval form of Amphistomum conicum (Paramphis- 

 tomum cervi). The other was also described by Looss (1896:177-185) 

 as the larva of Gastrodiscus aegyptiacus. This conclusion, however, only 

 rests on the structural comparison of the cercaria and the adult. 



The five amphistome cercariae now known belong to two different 

 sub-families of the Paramphistomidae. The cercaria of Paraphistomum 

 cervi differs from the others in lacking the pockets of the oral sucker, and 

 in having a connection between the longitudinal crura of the excretory 

 system. It belongs to the sub-family Paramphistominae. The other four 

 of these cercariae are much alike and belong to the sub-family Diplodis- 

 cinae. They all have the retrodorsal pockets of the oral sucker, and 

 the muscular enlargement of the esophagus at the bifurcation of the 

 intestinal ceca. Cercaria diastropha differs considerably from the other 

 three in its small size, lack of pigmentation and in the proportionally 

 large size of the oral sucker. The cercaria of Diplodiscus subclavatus, 

 that of Gastrodiscus aegyptiacus and Cercaria inhabilis are very similar. 

 The first of these differs from the other two in the large size of the 

 pharynx of its redia and in the small size of the intestine. Cercaria 

 inhabilis is larger than Looss' cercaria of Gastrodiscus aegyptiacus. My 

 measurements show that the oral sucker of this species is twice as 

 large as his, and the acetabulum is very much larger. There is noth- 

 ing in Cercaria inhabilis to correspond to the papillae found by Looss 

 around the mouth of his form, and the intestine of the redia is much 

 larger in his species. 



The only adult trematode which I have found in the literature 

 from the United States which resembles these cercariae in structure is 

 Diplodiscus temporatus Stafford. This form has the retrodorsal pharyn- 

 geal pockets found in the cercariae and also the muscular thickening 

 of the esophagus. The arrangement of the reproductive organs is such 

 that those of the adult might be derived from the anlage of either of 

 the above cercariae. As far as conclusions from comparative structure 

 are concerned either Cercaria inhabilis or Cercaria diastropha might 

 be the larvae of Diplodiscus temporatus. Infection experiments alone 

 can clear up this point. It is evident, that Cary (1909) is mistaken 

 in the larval form which he assigns to Diplodiscus temporatus. 



Cary (1909) describe^ as belonging to the life-history of Diplodis- 

 cus temporatus Stafford sporocysts and rediae, both containing cercariae 



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