SEA-FISHERIES LABORATORY. 225 



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 coelomic 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 oesophagus. 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 coelom. The 

 whole organ is essentially similar to the pronephros of 

 Lejndosteus 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. 



