60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
nematode probably attains maturity after passing from the body of the 
leech into that of another host, perhaps some fish, which feeds upon 
the leech. How the nematode gets into the body of the leech is likewise 
unknown, probably from the body of some snail or other small pond 
animal on which the leech feeds. 
The supposed trematode parasite I have observed but once, in Novem- 
ber, 1899, when three individuals were observed encysted in a single G. 
stagnalis. Unfortunately they with their host died in captivity before 
I had an opportunity to study them carefully. They lay imbedded in 
the deeper muscle layers of their host’s body, toward its anterior end, 
each enclosed in a delicate rounded cyst. A single ventral sucker was 
observed in the parasite and this seemed to lie a little nearer one end 
of the body. Toward the opposite end, a dark granular substance was 
observed in the interior of the body, probably in the digestive tube. 
My study of the parasite, was so incomplete that I should not feel war- 
ranted in asserting the absence of a second sucker more nearly terminal 
in position than the one observed. No measurements of the cysts were 
made, but I should estimate their diameter roughly at 0.50-0.75 mm, 
About half of the individuals of G. elongata which have come under 
my observation contain a gregarine which appears to be identical with 
that described by Bolsius (’96) as occurring in G. complanata (Clepsine 
sexoculata). I have not, however, made a sufficiently careful study of 
it to enable me to add anything to his account. I find the parasite 
attached always to the wall of the stomach diverticula (Figure 27, ga.), 
never in crop or intestine. 
A majority of the individuals of G. fusca collected by me contain 
sporozoa in an encysted condition. These parasites are quite common 
also in the body of G. heteroclita and that of G. elegans, and I have 
found them in a single individual of G. stagnalis. 
Whether or not they represent another stage of the gregarine found 
in G. elongata, I am unable to say. As already indicated, I have ob- 
served them only in stages of encystment, more or less advanced. One 
finds the heavily staining sporocyst in whole preparations of its host, 
usually near the margin of the body, imbedded in the deeper-lying 
muscle layers (longitudinal and dorso-ventral). The sporocysts which 
I have observed were spherical in form; the largest ones examined were 
about 0.13 mm. in diameter and were protected by a thick, dense wall. 
I have not yet been able to obtain sporocysts containing fully formed 
