STRUCTURE OF UROCHiETA AND DICUOGASTER. 273 
a sharp line of division between the Oligochseta and the Poly- 
chaeta. The peculiarities of the reproductive system will be 
the basis of this distinction. The investigations of Korschelt, 
Meyer, and Weldon upon Dinophilus have gone a long way 
towards demonstrating that this worm stands at the base of the 
Polychset series. Now, the nephridia of Dinophilus are in 
their minute structure comparable to those of the Platyhel- 
minths ; in most species they form a single pair of branched 
organs terminating in numerous “ flame-cells.” In D. gyroci- 
liatus, according to Meyer, each single nephridium is broken 
up into a series metamerically arranged, and each opening by a 
separate external pore. This I believe to be the way in which 
the Polychaet nephridia have arisen. 
There is no known form which seems to me to represent an 
intermediate stage between the Oligochseta and the Platyhel- 
minths. On the whole, it must be admitted that certain of 
the aquatic Oligochmta, such as the Naidomorpha, stand at 
the base of the Oligochset series. The fact that the nephridia 
of these Annelids are paired is a difficulty in regarding Peri- 
chseta as representing in the structure of its nephridia an 
ancestral form. It must be remembered, however, that our 
knowledge of the aquatic Oligochaeta, though no doubt fairly 
advanced as regards indigenous forms, is very small as regards 
exotic genera. Also there are traces (in An a ch set a, Vej- 
dovsky (29) (PI. VII, fig. 14) of what I believe to be the 
primitive condition. It may be that the (presumed) reduction 
of the nephridia in these aquatic forms has some relation to 
their small size, and, in consequence, to the reduced size of the 
coelomic cavities. 
It will be of no advantage to endeavour to combat Dr. 
Eisig’s arguments against regarding the nephridia of Acan- 
thodrilus multi por us as representing an archaic condition, 
principally for the reason that at the time when he wrote he 
was able to say that only one or two genera exhibited the 
dysmetameric condition, the vast majority having a metameric 
condition of the nephridia. 
We are now, however, acquainted with the following genera 
