36 ILLINOIS BIOLOGICAL MONOGRAPHS [482 



Islands from Melania baldwini Annecy and Melania newcombii Lea, 

 described briefly by Lutz (1893 :327). Since in these forms there are no 

 fixed larval organs such as spines, stylets, etc., to compare, specific dif- 

 ferentiation is difficult. Cercaria distomatosa, however, seems to show 

 some constant differences from Cercaria megalura. In Cercaria disto- 

 matosa the rediae reached a length of 1.8 mm., had a pharynx 0.07 mm. 

 long, and a digestive system extending only to the posterior locomotor 

 appendages, which are about three-fourths of the distance from the 

 anterior to the posterior end. Also rediae were found in which rediae 

 were developing. In Cercaria megalura as noted above the digestive 

 system was more voluminous and reached almost to the posterior end of 

 the body, and the posterior locomotor appendages were further back, 

 being about six-sevenths of the distance from the anterior to the posterior 

 end. Also Looss' measurements for Cercaria distomatosa are slightly 

 larger than those for Cercaria megalura, and the judge from his figure 

 and description, the tail is considerably smaller. He found the final tub- 

 ules of the excretory system in the tail, which were not found in my 

 species. The Hawaiian cercaria described by Lutz (1893:337) seemed to 

 be similar to Cercaria megalura and Cercaria distomatosa in having a 

 long tail by which it can become attached, in the general relationship of 

 the body structures, and in the form of the cyst. A more detailed com- 

 parison is impossible on account of the meagerness of Lutz's description. 

 It is evident that these three forms are very closely related. Cary's 

 attempt to relate Cercaria megalura to Diplodiscus temporatus is the 

 only suggestion as to the life-history of a member of this group, but as 

 shown above, this cannot be accepted. 



ECHINOSTOME CERCARIAE 



Cercariae belonging to the family Echinostomidae are very easily 

 recognizable on account of their structural correspondence to the adults. 

 It is impossible to subdivide them into smaller natural groups. The 

 following may be given as a brief statement of characters for the 

 Echinostome cercariae. 



1. Distome cercariae developed in rediae. 



2. Rediae have collar, birth-pore, and posterior locomotor append- 

 ages. 



3. Cercaria with digestive system consisting of prepharynx, 

 pharynx, long esophagus, and intestinal ceca reaching the posterior end 

 of the body. 



4. Anterior end of cercaria with collar and circle of spines. 



