174 ROBERT W. HEGNER. 



In the latter the abdominal body wall allows the light to pene- 

 trate to the intestine more easily than in the toad tadpole, which 

 is characterized by the presence of dense black pigment. The 

 nutrition of the euglenoids in the toad tadpoles thus becomes 

 almost entirely by absorption whereas in the tadpoles of Rana 

 pipiens it is still partly holophytic. It is interesting to note that 

 Species A is sensitive to light, congregating on the side of the slide 

 toward a north window and moving from one side of the slide to 

 the other, a distance of 16 mm. in about 20 minutes, when placed 

 opposite this window. 



VII. Infection of Toad Tadpoles with Species A by Association 

 with Infected Tadpoles of Rana pipiens. Evidence was presented 

 above that tadpoles of Rana pipiens can be infected with eugle- 

 noids of Species A by feeding them food containing active tropho- 

 zoites, but this probably is not the method of infection in nature. 

 Inasmuch as all tadpoles in certain ponds were found to be in- 

 fected and none or a very few in other ponds, infection by associa- 

 tion was suggested. To test this method the following experi- 

 ment was carried out. A large number of toad tadpoles were 

 collected on July 10. Five of these were examined on July 23 

 and found to be uninfected. Seven of the remaining tadpoles 

 were then placed in a finger bowl with 7 tadpoles of Rana pipiens 

 taken from a lot that were all infected. They were kept together 

 for five days. Three of the toad tadpoles were still alive, three 

 had recently died, and one had died previously and been removed. 

 Each of the three living toad tadpoles, on examination, was 

 found to contain large numbers of Species A in the rectum and a 

 few in the intestine. No specimens were found in one of the dead 

 tadpoles but a few were present in the other two dead tadpoles. 

 At this time ten more toad tadpoles from the control lot were 

 examined; eight of these were uninfected and one specimen of 

 Species A was found after careful search in the rectum of each 

 of the other two. These results prove that uninfected tadpoles 

 may become infected by associating closely with infected tad- 

 poles. The obvious way in which this is brought about is the 

 escape of specimens from the rectum of infected tadpoles into 

 the water and their ingestion by the uninfected tadpoles. This 

 probably occurs in nature only when the tadpoles are closely 

 associated. Toad tadpoles are very gregarious and might easily 



