52 ILLINOIS BIOLOGICAL MONOGRAPHS [498 



Only one of the forked-tailed cercariae, Cercaria ocellata La Valette 

 St. George corresponds at all closely in structure to Cercaria douthitti. 

 The structure of this form has been fairly well worked out by La Valette 

 St. George (1855:22-23) and Moulinie (1856:172-173). Cercaria ocellata 

 agrees with Cercaria douthitti in the large unicellular glands of the pos- 

 terior body region, in the presence of eyespots, in the length and jointed 

 character of the tail, and in fact that the forked portion is only one-third 

 of its total length. The ratio in size of the suckers may agree as stated 

 above. 



Cercaria ocellata differs in several particulars from Cercaria douth- 

 itti. The former is almost twice as large, is found in a different host, 

 in a different continent, and has narrow fin-like extensions on the divided 

 lobes of the tail. 



No suggestion can be made as to the life-history of Cercaria douth- 

 itti. Its structure is such as not even to suggest to what family of dis- 

 tomes the adult belongs. In fact hardly a suggestion has been made in 

 regard to the life-histories of the forked-tailed cercariae and no experi- 

 ments that I can find have been carried on to trace their development. 

 Certainly further studies are needed on their structure and develop- 

 ment. 



XIPHIDIOCERCARIAE 



Liihe (1909:189) defines Diesing's (1855) group of the Xiphidio- 

 cercariae or stylet cercariae as follows : 



Slender-tailed distome cercariae with a boring spine on the rounded 

 anterior end. Eyes lacking ; develop in sporoeysts ; encystment in a sec- 

 ondary intermediate host. 



Since this group is formed on likenesses in but few larval char- 

 acters it can be considered only within wide limits as expressing rela- 

 tionship. On account of their small size and also since many of them 

 are known only from the older accounts many of the forms of this group 

 are very insufficiently described. Five new forms are added to this 

 group by the study of my material. In the following account the new 

 American forms will be compared with the most closely related of the 

 already known species, and where it is possible an attempt will be made 

 to fit them into natural groups. 



Two of these forms with the related European species agree so 

 closely in structure that a new group, the Polyadenous cercariae will 

 be formed for them. 



