430 WM. CECIL BOSANQUET. 
host seems all the more likely from the fact that the cysts 
occur in the same masses of tissue as the nematodes, which 
are known to act in this manner. 
Spores of two Sizes.—It is also of interest to note that 
there appear to be two different kinds of spores occurring in 
similar positions in cysts which are also similar, one being a 
large variety measuring approximately 30 pw in length by 12 u 
in breadth, while the other is almost exactly half this size, 
viz. about 15 w by 5 mw (fig. 19). The description of spore- 
development given above applies in the main to the larger 
variety which are more easily observed, but, as far as I have 
been able to make out, in the case of the smaller kind very 
similar changes occur. The size of individual cysts differs 
considerably in both kinds, but the cysts containing the smaller 
spores are not themselves necessarily smaller than those in 
which the large variety is found. I have found in one or two 
instances a cyst containing large spores surrounded by tissue, 
in which were embedded, apparently loose, a number of the 
smaller kind. In cutting some sections through portions of 
the vesiculz seminales of a worm, I also found cysts containing 
spores of two different sizes, corresponding very closely with 
the two varieties found in the hinder end of the worm. It 
would be natural to suggest that these two varieties in the 
vesiculz are respectively the spores of Monocystis magna 
and agilis, and that the spores in the posterior segments may 
also belong to these gregarines, the large round gregarines 
described being a form of M. magna. Against this theory, 
however, stands not only the distance from the known haunts of 
M. magna and agilis, but also the large size of the cysts of 
small spores, which could hardly be formed by M. agilis, 
while the cysts in the vesicule containing large spores were 
themselves no larger than those containing small ones ; and it 
seems necessary to suppose, in default of further evidence, 
either that two sorts of spores are formed by each gregarine, or 
that still another species of gregarine remains to be described. 
It therefore appears best for the present to consider the round 
gregarine of the tail segments as distinct from the other 
