81 
nectids the segmental ducts originate as solid rods, which 
afterwards acquire a lumen by the radiate arrangement of 
their cells. This condition, however, is most probably a 
secondary one, and the mode of development as a longi- 
tudinal groove seems most primitive. The cavity of the 
paired ducts of the adult kidney and that of the tubules is 
accordingly celomic in its nature. During larval life 
these ducts are the efferent channels of the pronephros— 
the larval excretory organ. 
The Pronephros is probably formed before the larva 
hatches from the egg. Text-fig. 2 represents the condi- 
tion of the organ in a plaice 12 days old. It is part of a 
transverse section through the anterior part of the trunk 
immediately behind the gill-bearing region. Here the 
segmental duct makes two or three convolutions and opens 
by a non-ciliated nephrostome into a small chamber. 
The right and left pronephric chambers lie side by side, 
separated by a thin septum. The dorsal aorta lies between 
them in the dorsal thickened part of the septum. A 
vascular tuft, the glomus, projects from the lateral wall 
of the aorta into each pronephric chamber opposite the 
nephrostome. The whole organ lies between the noto- 
chord and the csophagus. It has no connection, at 
least in the stage studied, with the body cavity, but 
there can be little doubt that the pronephric chamber is 
simply an enclosed portion of the general celom. ‘The 
whole organ is essentially similar to the pronephros of 
Lepidosteus as described by Balfour and Parker, except 
that in the latter form the pronephric chamber still com- 
municates with the body cavity by a richly ciliated funnel. 
The glomus is really a tuft of capillaries in communica- 
tion with the aorta. ‘The resemblance of the whole struc- 
ture to a Malpighian body of the kidney with its contained 
glomerulus will be noted. 
I 
