614 THE AMERICAN NATURALIST. |. (Vor. XXXIX. 
Its implication is that the relationship between the Polycystidea 
and Monocystidea is very close. Relatively, this is true, yet 
there are considerable differences. For one thing, reproduction 
in the Monocystids is isogamous. The encysted trophozoites 
each break up into gametes, which are allalike. These gametes 
` fuse in pairs to form the zygotes. It is assumed that in each 
zygote, one gamete has been derived from one trophozoite and 
the other from the other, but the point is necessarily almost 
impossible to demonstrate. Each zygote produces a spore. 
In the second place, the Monocystidea are far simpler in 
organization than the Polycystidea. While some of them 
possess a definite body form, many do not. Moreover, as their 
name indicates, their bodies are never divided into two chambers, 
a result of the loss of the sarcocyte. It is on this same account, 
in all probability, that they are polymorphic. Now, throughout 
the Polycystidea a perfect series can be established based on the 
development of the sarcocyte. In some it is always thick and 
continuous all over the body. In others it is but feebly devel- 
oped, while in a certain number the septum disappears, and these 
species simulate the Monocystidea. The element is obviously in 
a transitional state, and since we are dealing with parasites, we 
are warranted in supposing it is disappearing. 
Considering next the life history, the Monocystids, like the 
Polycystids, develop from a sporozoite which is released in the 
Intestine of the host. But whereas in the latter group the 
SPorozoite never gets farther than the intestinal epithelium, 
In the former it penetrates the coelome. Some Monocystids 
develop in the connective tissue surrounding the intestines of 
their hosts, whereas others go farther. Thus, in the Monocys- 
tids of the earthworm, the sporozoites gain the seminal vesicles 
before developing into gregarines. 
We are thus furnished with certain data enabling us to deter- 
mune which of these two groups is ancestral. The evidence 
awards this distinction to the Polycystidea. That furnished by 
the Sarcocyte is very suggestive, and the Monocystidea, which 
lack It, are to be considered the derived group. The life history 
points in the same direction. It is to be assumed here, as it is 
in the case of most animals, that ontogeny recapitulates phy- 
