178 ROBERT W. HEGNER. 



ing to Alexeieff, is a case of accidental parasitism which is a stage 

 in the evolution of parasitism among the Euglenoidina. He 

 accounts for the presence of these euglenoids by the mode of 

 nutrition of the tadpoles, which engulf large quantities of debris 

 of all sorts resulting in a thick, compact mass in the intestine. 

 Orgarfisms in the midst of such a mass might easily escape the 

 action of the* digestive juices, especially since these are greatly 

 diluted, and as a result become gradually acclimated and finally 

 facultative parasites. 



XIV. Summary and Conclusions. -(i) Three species of Eugle- 

 noidina are described from the intestine and rectum of frog and 

 toad tadpoles; all three species possess green chromatophores and 

 bright red stigmas. 



(2) A comparative study of two sets of tadpoles of Rana pipiens 

 from adjoining ponds, one set much retarded in growth and 

 heavily infected with Species A and the other of normal growth 

 but lightly infected or not at all, indicates that the dwarfing of 

 the former may have been due to the presence of euglenoids. 



(3) Tadpoles of Rana pipiens infected with Species A were 

 kept in the laboratory without food for 25 days and specimens 

 examined at intervals. The infection persisted throughout this 

 period without any marked decrease in the brightness of the 

 green color of the euglenoids. Only one case of division was noted, 

 and no cysts were found, the euglenoids remaining as trophozo- 

 ites, free swimming and actively metabolic, throughout the 

 period. The retention of the green color may have been due to 

 the transmission of light through the almost transparent abdomi- 

 nal and intestinal wall. The rectum is the usual habitat of this 

 species but the intestine is often invaded especially the first 10 mm. 

 just anterior to the rectum. They are not mixed with the intestinal 

 and rectal contents but move about between this mass and the 

 containing walls. Infection was found to persist in tadpoles 

 collected from time to time from the pond. 



(4) Tadpoles of Rana pipiens containing very few specimens 

 of Species A were fed on the intestines and recta of highly in- 

 fected tadpoles of the same species. A great increase in the 

 number of Species A in the experimentally fed tadpoles proves 

 that infection with this species can be brought about by the 

 ingestion of active trophozoites with the food. This, however, 



