494 HARRY M. MILLER, JR., AND FLORA E. NORTHUP. 



relatively little is known (Meek, 1916; Bigelow and Welsh, 1925). 

 Why a certain percentage of infestation, such as the maximum 

 of December or June, once reached is not maintained would seem 

 to be due directly to two factors, the death of heavily infested 

 snails, and the recovery of some of the snails because of the 

 maturing and complete emergence of the larval trematodes. 

 The serious and often fatal effects of larval trematode parasites 

 on the snail host have been studied by a number of investigators. 



total 

 infcslvtion 



Aug 



Oct 



Dec Jan 



Feb 



Mar Apr way June July ftui) 



TEXT-FIGURE 2. Cercariaum lintoni; graphs showing total infestation and per- 

 centage of emerged (mature) cercariae. 



The very rapid increase in actively metabolizing trematode tissue, 

 which frequently results in the destruction of most of the visceral 

 mass of the snail, probably causes the death of large numbers of 

 them. As to recovery, Sewell (1922: 17) and others have noted 

 a condition of degeneration of liver or gonad apparently at- 

 tributable to previous trematode infestation; the present author 

 has also found this condition in some individuals of each of a 

 number of species, such that at first glance the liver appeared to 

 be parasitized, but no trematodes were present. Sewell included 

 also the factor of natural death at certain seasons of the year 

 as one which might explain seasonal fluctuations in percentage of 



