364 MONTGOMERY. [Vor. XII. 
nephridium, but rather it must be compared to a system of 
organs, since a number of different functions are performed by 
it, and since it stands in a peculiar relationship to most, if not 
all, of the bodily organs. The whole origin, manner of differ- 
entiation, and extent of development of the body cavity, is to a 
great extent dependent upon the mutual positions of the organs 
as well as upon their degree of correlation. Therefore, any 
change which causes a difference in the relative positions of 
the organs must also effect some degree of change in the 
diversification of the body cavity. 
The body cavity being essentially a space or system of spaces 
separating and penetrating the organs of the body, a common 
possession of these organs, cannot be correctly homologized, for 
the very fact that it is not comparable to any individual organ. 
It would be as difficult to satisfactorily homologize these 
cavities in different forms as to establish homologies between 
states of correlation, or between mutual arrangements of 
parts. And in case examples are required we may consider 
the group of the Nemerteans, for it was a study of the body 
cavities in this interesting group of worms which led me to 
make the present examination into the question of possible 
homologies. 
In the Nemerteans, the body cavity occurs as (1) the rhyn- 
chocoel (the large space surrounding the proboscis); (2) the 
cavity of the blood-vessels ; (3) the perivisceral space situated 
between the intestine and blood-vessels on the one hand and 
the body muscular wall on the other. The portions of the cav- 
ity mentioned under (2) and (3) are remnants of the cleavage 
cavity (z.e., archicoelic in origin); but whether the rhynchocoel 
also is archicoelic has not yet been definitely settled. Now in 
a previous contribution, to which I have already referred, I com- 
pared the pseudocoel in this group to the coelom of the Annelids, 
and for the following reasons: the Nemertean pseudocoel — 
that space the existence of which has, until recently, been ques- 
tioned —encloses in the adult worm true mesenchym tissue con- 
sisting of multipolar cells ; it is from certain of these cells that 
the sexual cells are derived. Further, in all the species where 
the gonadal sacks are not preformed, —and this is the case in 
