86 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Data obtained more recently from examination 

 of 189 naturally infected drills collected in all 

 seasons from 1956 through 1959 require revision 

 of my earlier (1957) brief description of the pa- 

 thology of drill infection. 



The digestive gland of the uninfected drill is 

 variable in color, usually light gray or beige, but 

 may be very dark, almost black; it has a soft, 

 cheesy consistency, and is covered by a fairly 

 tough tunica propria. The digestive gland of the 

 infected drill is less variable in color, usually 

 lighter gray or light yellow, is swollen, has a softer 

 consistency and a more easily toni tunica propria, 

 through which can be seen enormous nimibers of 

 rediae scattered among widely separated small 

 masses of host tissue. 



The gonads of uninfected drills vary in size and 

 tliickness with seasonal changes in reproductive 

 activity, but the color is usually brownish in males 

 and yellow to orange in females. In infected drills, 

 the gonad is thin and patchy, or completely ab- 

 sent; when present, its color is cream to brown in 

 males and usually yellow to orange, sometimes 

 brownish, rarely oyster white, in females. 



Sex determinations were made on approximately 

 one-half of all infected snails (98 of 189) collected 

 in the present study ; the sex ratio of these snails 

 was approximately one male to 1.6 females. It 

 is found, even in small lots of drills, that a 1 : 1 sex 

 ratio usually prevails in field collections. 





s-W' 



Figure 7. — Daughter rediae in various stages of development in ovary, natural infection; Z, DH-AzB-E ; X212. 



