102 Mr. F. II. Stewart on the 



and .r ; while the nephridial tube runs along the branch {y) 

 which passes to the vascular tuft {c.p ) (not figured by 

 Goodrich), and does not in any part of its course touch the 

 dorso-ventral. 



The Excretion of Solid Matter and the Function of the 

 Ciliated Organ. 



In examining under the microscope a nephridium which 

 lias been carefully dissected out from a hardened specimen, 

 the nephridial canal will be seen as a conspicuous green tube 

 lying along the blood-vessel y (PI. II. fig. 2) on its outer and 

 posterior side. Up tliis blood-vessel it runs until it reaches 

 a point opposite the base of the ciliated organ ; here it bends 

 sharjjly round in front of y, passes across to the ventro-lateral 

 branch, and forming a semicircle round this, it appears on the 

 posterior and inner side of the ciliated organ, where it termi- 

 nates in the solenocyte- bearing tuft (PI. II. fig. 2, s.t.). 



It is the portion of the tube between the ventro-lateral and 

 y that demands special attention, for here alone is it in close 

 contact with the ciliated and grooved side of the ciliated 

 organ. Indeed it here forms a miniature barrier at the exact 

 point where the grooves and the streams induced by the ciliary 

 action converge (figs. 3 & 4). 



The importance of this disposition is extremely well illus- 

 trated by placing a living nephridium in a drop of sea-water 

 laden with carmine particles. The red grains may be seen 

 carried by the currents down the grooves of the ciliated organ, 

 and deposited against the barrier of the nephridial tube until 

 a solid mass is formed. 



Again, I observed that the yellow-green coloration caused 

 by the presence of excretory matter in the walls of the tube 

 extends only as far as the ventro-lateral vessel {b, PI. II. 

 fig. 3), and is not continued up to the solenocyte-bearing tuft. 



These considerations suggested that it would be in the 

 short stretch of the tube between the ventro-lateral vessel and 

 y that solid excretory matter would be taken up. 



The following facts appear to confirm this view : — 



(1) When a nephridium is extracted it may be noticed 

 that there is always present against this barrier a mass of 

 phagocytes from the coelomic fluid filled with yellow-green 

 excretory matter (figs. 3 & 5). These are evidently carrying 

 waste products to this part of the tube, and I have actually 

 observed one of these cells entering the wall at this point 

 (PI. III. fig. 4,;>). 



