1920] Joh nson : Life < 'ycle of Echinostoma Rcvolutum (Froelick) 363 



This encysted agamodistome is riot specific to Physa occielt it talis 

 since once I found it in Lymnaea trashi. Also out of twenty plana riae 

 taken from the same rocks as the snails, six contained cysts, one having 

 seventeen embedded in its muscular tissue and another ten. The other 

 four, however, had only one, two, or three cysts. Leeches taken from 

 the same rocks gave negative results, as did the tadpoles of Hyla 

 regilla Baird and Girard, and Notophthalamus torosus (Rathke), into 

 whose aquaria active cercaria were placed. I predict that a thorough 

 canvass of Stow Lake or other places where this cercaria is found would 

 reveal other hosts for this stage. This lack of specificity of the 

 encysted agamodistome is quite common in digenetic trematodes. 



There are at least three ways in which the cyst may be taken into 

 the primary host. The first and perhaps the usual way is when in- 

 fected snails are eaten by ducks, geese, etc. McAtee (1918) reports, 

 after a careful study of the food habits of seventeen hundred and 

 twenty-five mallard ducks of the United States, that 9.47 per cent of 

 the food consists of animal matter. He says, "Mollusks, the most 

 important element of animal food of the mallard, comprises three- 

 fifths of this and 5.73 per cent of the total. Fresh water snails are 

 represented most numerously, no fewer than fifty sometimes being 

 taken at a single meal." That the American scaup duck, Marila 

 marila (Linn.) eats snails is certain, but the exact amount has, so far 

 as I know, never been tabulated. The mallard ducks eat over 90 per 

 cent vegetable food in the wild, but it is likely that a still larger per- 

 centage is of vegetable matter in parks where they are fed daily. For 

 this reason I should expect to find ducks living on ponds where addi- 

 tional food is not supplied and where the echinostome infection of 

 snails is equally heavy, to be more parasitized with the echinostome 

 adults than those fed in a park. 



Dead snails also are probably eaten by ducks, geese, etc. Hundreds 

 of specimens of Physa occulcntalis were seen dead on the surface of 

 Stow Lake. Since this water is not used for drinking purposes, and 

 therefore no chemicals are used, and because there were also many 

 healthy active snails, I feel sure that their death was caused largely 

 by the echinostome parasites. The dead snails were found upon exam- 

 ination to be heavily parasitized in nearly every case. Sometimes the 

 infection was so heavy that in quantity it was about one-third the size 

 of the viscera of the snail. If only cysts were present, little harm 

 would probably result, but with an abundance of rediae absorbing 

 food and giving off wastes into the body, considerable injury is bound 



