1920] joh nson ; Life Cycle of Echinostoma Bevolutum (Froelich ) 341 



revolutum. The greatest length yet recorded for this species is by 

 Looss (1899, p. 682), who has obtained the extreme measurement of 

 0.134 mm. Throughout the whole genus Echinostoma, however, the 

 mean is not far from 0.110 mm. The great variation in size of eggs in 

 Echinostoma revolutum suggests that Looss (1899, pp. 679-684) and 

 others, have placed several species under this one name. This will be 

 more fully discussed under the heading of the adult echinostome. 



INFECTION OF SNAILS AND DEVELOPMENT OF 

 MOTHER^-REDIAE 



Young snails of Physa occidentalis were placed in Syracuse dishes 

 with the eggs of Echinostoma revolutum in which the miracidia were 

 almost fully developed. These young snails were known to be free 

 from any trematode infection since they were raised in small aquaria 

 from eggs produced during the months of August and September, 

 1918. To make doubly sure, many of the young snails were killed and 

 carefully examined but all these examinations gave negative results. 



At the time of the first experiments the snails were about six 

 months old. Usually eight snails were placed in each Syracuse dish 

 with about fifteen eggs. During the first day almost all the eggs were 

 eaten by the snails, but they passed through the digestive tract of the 

 snails unharmed. They remained in the feces until they hatched, 

 which occurred at the end of twenty-one days. 



In one Syracuse dish in which no young snails were placed, the 

 movement of the miracidia outside the egg was studied. Their activity 

 is much like that of other miracidia for about twenty-four hours. At 

 the end of that time the vigorous activity quite rapidly subsides, 

 followed in a short time by a breaking of the body wall and the 

 extrusion of some of the contents. 



In the other Syracuse dishes the miracidia must have penetrated 

 the young snails almost immediately since they were never to be found 

 free in the water except for a few minutes after hatching. The 

 penetration of the young snail by the miracidium I was never fortunate 

 enough to observe. Because of its large anterior papilla, which is 

 capable of being greatly retracted and then forcibly extruded, I judge 

 that it would encounter little difficulty in piercing the thin body wall 

 of the viscera or even the more rigid foot of the snail. 



From time to time, these young snails were carefully examined to 

 see what stage followed. All the evidence points to the fact that the 



