494 F. ERNEST WEISS. 



to excrete over the greater portion of their epithelial struc- 

 tures. 



But the excretion of this atrial surface is not comparable in 

 quantity, at least in the excretion of carmine, with the amount 

 excreted by some small tubules which have hitherto remained 

 unnoticed, but become very evident in the specimens fed with 

 carmine. The course of one of these tubules is figured in the 

 drawings of the successive sections (figs. 1 to 5). They lie on 

 the outer side of the topmost of the pharyngo-pleural folds 

 which connects the uppermost primary bar with the lateral 

 ridge, from which all the gill bars start. Along this ridge we 

 find a continuous chitinoid rod, which is connected in turn 

 with each rod of the series of bars. These tubules occur thus 

 serially, the last one being in connection with the last primary 

 bar, and lying therefore much more ventrally than the fore- 

 most ones. They seem to project into the coelomic cavity, but 

 at the same time are covered with the thin coelomic epithelium 

 which lines that cavity. 



In fig. 1 the tubule is cut longitudinally, and on its inner 

 side will be seen a blood-vessel which is a branch from the 

 dorsal aorta, and passes over the longitudinal chitinoid rod of 

 the lateral ridge to join the aortic trunk. This branch of the 

 aorta is seen passing upwards in figs. 3, 4, and 7. It is from 

 this branch, too, that the blood- supply to the pharyngo-pleural 

 folds and the vessel along the outer edge of the chitinoid rod 

 of the gill bar is derived. 



From the subsequent sections it will be seen that the tubule 

 runs upwards and backwards, and then bending downwards 

 opens (figs. 4 and 8) into the upward extension of the atrium 

 at the highest point, which the latter reaches between the 

 pharyDgo-pleural folds. The opening is always placed at the 

 origin of a secondary gill bar, and is just the place at which 

 an excreting organ might profitably open to ensure its excreta 

 being carried away. The chief difficulty with regard to the 

 making out of these points was the obtaining of a suitable 

 stain which would not hide the carmine granules. Carmine 

 and haematoxylin proved quite unsuitable, and after trying 



