364 University of California Publications in Zoology [Vol. 19 



to be done, and in extreme eases I believe death results. Lebour (1912, 

 p. 423) makes this statement concerning the effect of the parasite on 

 the host: "The presence of sporocysts and rediae certainly injures the 

 molluscan host. The outer appearance of the digestive gland is usually 

 enough to show if it is inhabited by these parasites. It looks unhealthy 

 and is a grey, sickly yellow or a peculiar unnatural orange. It is 

 generally completely riddled with the sporocysts or rediae which feed 

 upon its substance. If the gonad is infected it is quite destroyed. On 

 the other hand encysted cercariae seem to do little or no harm to their 

 hosts, even though present in large numbers." From my own obser- 

 vations, and from statements by other workers. I am inclined to believe 

 Lebour is right. During the fall of the year dead snails are most 

 abundant. This fact is significant in light of the fact that the rediae 

 and cercariae are also most abundant at this time. The snails detach 

 themselves at death from the rocks and float to the surface, having 

 nearly always a large part of their body extruded from the shell and 

 thus they are very accessible as food to the water birds. Probably in 

 this way many birds are infected. 



Another method of infection is that cysts are scooped up from the 

 bottom of the pond by ducks, geese, etc., in their search for food in 

 .shallow lakes. Rediae and cercariae seldom live more than twenty- 

 four hours in a dead snail, but the encysted agamodistomes are known 

 to live through the decaying of the snail apparently unharmed by the 

 putrefying process. These cysts then sink to the bottom of the pond, 

 where they remain alive for a considerable time. A somewhat similar 

 condition is illustrated by the behavior of the cysts of Paragonimus 

 ringeri which break loose from the gills of the fresh water crabs and 

 crayfish and sink to the bottom of the pond or stream. 



Still another method is that of ducks, geese, etc., eating planaria 

 and possibly other secondary intermediate hosts. However, because 

 of the prevalent infection of this snail with the cysts (70.5 per cent) 

 I feel that the planaria and other forms of animal life play a minor 

 role in the infection of the host of the adult eehinostome. 



The cyst wall is not hard or impervious to water, as was proved by 

 accident when 5.8 per cent salt solution was used instead of normal. 

 In each case within three minutes every cyst, and of course every redia 

 and cercaria, was dead. This probably means that the cyst is absorbing 

 moisture all the time and possibly a small amount of food. The cyst 

 wall is usually round, although it is capable of being changed in shape 

 slightly by the movement of the enclosed agamodistome. 



