483] LARVAL TREMATODES—CORT 37 



5. Excretory system opening on each side of the anterior part of 

 the tail ; excretory bladder small, crura large, reaching to oral sucker. 



6. Tail powerful, longer than body. 



In the material used in this study were two echinostome cercariae. 



The first species was found in several specimens of Planorbis trivol- 

 vis, examined during November, 1913, from a small pond near Urbana, 

 Illinois. The infection consisted of rediae containing cercariae in the 

 livers, and encysted cercariae in the body cavities of the snails. Planor- 

 bis trivolvis is then able to serve both as intermediate and secondary 

 intermediate host for this species. I propose for this cercaria the name 

 Cercaria trivolvis. The second of these larvae was found in a few out 

 of thirty-six specimens of Campeloma subsolidum from Hartford, Con- 

 necticut. The snails in this case I regard simply as the secondary 

 intermediate host since no rediae were found. From the fact that the 

 encysted cercaria had a pinkish tinge produced by pigment granules 

 in the postacetabular region the name Cercaria rubra is proposed for 

 this species. 



Cercaria trivolvis (Fig. 39) completes its development before leav- 

 ing the redia. Therefore very few cercariae were found free in the 

 liver of the host. That a certain time is spent in free life is suggested 

 by the fact that altho free swimming cercariae were kept under observa- 

 tion for a whole day none were seen to encyst. There seemed to be no 

 connection between infection in the liver and cysts in the body cavity, 

 since altho a few snails had both types of infection the majority had 

 only one. 



This cercaria (Fig. 39) moved actively both in open water and on 

 a substratum. The tail was powerful and extended when the animal 

 was swimming to two or three times the body length. For the swimming 

 movement the cercaria bent ventrad almost double, with the posterior 

 half of the body almost directly dorsad of the anterior. The tail which 

 extended beyond the anterior end lashed vigorously and propelled the 

 animal rapidly. When a cercaria came in contact with a surface it 

 took hold with its suckers and moved actively with a creeping movement 

 similar to that already described for Cercaria megalura. The structure 

 and position of the crown of spines suggests that it would be of consider- 

 able aid to the animal in making its way thru connective tissue. 



The cysts of this species were oval, having an average length of 

 0.16 mm. and width of 0.15 mm. The cyst wall varied in thickness 

 from 0.007 mm. to 0.012 mm. 



Cercaria trivolvis (Fig. 39) is elongate, pointed anteriorly, and has 

 its greatest width in the region of the acetabulum. It has an heart- 



