38 ON THE ANATOMY OF MEGASCOLIDES AUSTRALIS, 
Platyhelminth type, yet the development of the two groups has proceeded, starting 
from the same point, along somewhat divergent lines. 
The difference is due to the greater development of the ccelomic space in the 
Cheetopoda than in the Hirudinea, and the more complete division of the body of the 
former by means of septa into a series of compartments, each almost completely 
separated off from the ones on either side of it. There is at the present time a strong 
contrast in the two groups in this respect, and it is one which has probably held true 
since the time at which each group separately branched off from common 
Platyhelmunth-like ancestors to pursue its own course of development. We may 
believe this to be true, even though at the same time we agree with Bournz* that the 
ccelom of the leeches has once been more highly developed than it is now, and that 
“the leeches have thus had an ancestor which, in possessing a ccelom, was already a 
great advance upon any Platyhelminth form.” 
It was the development of this definite space in the mesoderm, which resulted in, 
or at all events was closely connected with, the formation of the definite internal 
openings of the nephridia in the leeches and, as I would suggest, of the present 
nephridial tubes of the Chetopoda. 
1.—The Hirudinea.—The simplest form of nephridial system known in leeches is 
that of Branchellion. As described by A. G. Bournz,t this consists only of a 
network of tubules, with no internal openings, and only a single pair of posteriorly 
placed external openings. The only indication of segmentation lies in a slight 
crowding of the tubules together at intervals. There can be little question of the 
close relationship of this to the system as found in the Platyhelminthes, and it might 
possibly be that further examination of the living form would result in the finding of 
flame cells. 
In Pontobdellayt a similar network is present, but there 1s in addition a segmental 
arrangement clearly indicated by the presence in each segment of an internal pair of 
ciliated funnels, and also of a pair of external openings. The former open into a 
definite coelomic space on each side of the body. 
The resemblance between the network, again, of Pontobdella and a Platyhelminth 
is close; the difference between the two lies in the presence of the two sets of 
openings. It must be noted that Lane has shown that a definite, though rudimentary, 
metameric arrangement of the nephridial tubes obtains in some planarians in which 
secondary external openings may be developed by means of branches from the 
longitudinal trunks, and that a paired arrangement of these may even be seen. The 
ciliated funnel is united to the network by a very short tube. Its development, as 
above suggested, may be supposed to be connected with the presence of the definite 
Op. cit. BP, 499. + Op. cit. P. 481. 
t Op. cit, BP. 478. 
