EMBRYOLOGY OF BDELLODRILUS 185 
X and X on either side (figs. 50-54). Both cells are nephrid- 
ial. This fact perhaps is made more suggestive by the behavior 
of X° and X, Either of these cells may divide equally, but 
never both in the same embryo. In either case we have three 
teloblasts derived from the nephroblast X‘? on either side. 
The cells of the nephridial rows are somewhat smaller and 
narrower than those of the neural rows. In some cases the 
outer. nephridial row of cells is very short. In other embryos 
it is composed. of but one or two cells and its presence is hard to 
verify, suggesting a possible disappearance in the group. As 
stated above, the nephridia arise in connection with a con- 
tinuous nephric cord of ectoblastic origin, which forms a part 
of the middle stratum of the germ band and lies along side of 
the neural row. Each nephric cord terminates at the posterior 
end in three teloblasts. Thus the entire nephric cord of cells 
is formed by the continued division of the ‘nephroblasts,’ which 
agree precisely with the neuroblasts in structure, action and 
mode of origin. The nephric cord at first is composed of three 
rows of cells posteriorly, but passing forward the rows are no 
longer definitely separated and the nephric cord or plate con- 
sists of an irregular series of cells which extend anteriorly to the 
posterior end of the pharynx. The formation of the nephridia 
progresses from in front backwards and keeps pace with the 
formation of new segments in the embryo. The beginnings of 
a pair of nephridia are found in each of the main segments. 
Only two pairs of nephridia are retained in the adult worm. 
The details of the formation of these segmental organs have not 
been worked out. 
Berg considers the entire nephridia in Criodrilus as mesodermal 
in origin; Whitman held the extreme opposite view, that the 
entire nephridium was ectodermal in origin; while Wilson re- 
garded the nephridia as being part mesodermal and part ecto- 
dermal in origin. In Bdellodrilus the nephridia are ectodermal. 
The anterior pair occurs in the first, second, third and fourth 
body segments. The left nephridia of the anterior pair, extends 
from the first to the third segments inclusive, while the right 
extends from the second to the fourth segments inclusive. Both 
