ON THE DEVELOPMENT OF LINGULA ANATINA. 63 



was enclosed in the body wall at the 5-6 p. c. stage, is now 

 constricted off from, but on one side is still apposed to, the body 

 wall (PL VIL, Fig. 110). We see that the size of the cells 

 forming the sheath varies considerably according to their position. 

 In a section of the funnel a ring of cells with deeply stained 

 nuclei is seen (PI. VIIL, Fig. 134). If traced posteriorly, the 

 funnel will be found to open dorsal ly to the body cavity ; the 

 ventral lip of the funnel is produced, forming part of the ileo- 

 parietal band. The left nephridium is usually closely apposed to 

 the ventral body wall, though no cellular connection with the 

 latter exists, as is proved by the fact that the nephridiun is 

 sometimes found widely separated from the body wall. The right 

 nephridium, on the other band, is separated in its posterior half 

 from the body wall by the intestine. And we find that it passes 

 posteriorly closely apposed to the dorsal surface of the intestine 

 (PI. VIL, Fig. 104, and PI. VIIL, Fig. 134). 



During free-swimming larval life the axis in the tube part 

 of the nephridium is, I think, the active organ in excreting the 

 waste material which is finally ejected to the exterior through 

 the central canal. In excretory function the sheath must also 

 play an important role in accumulating and transmitting the 

 waste material to the axial tube. In what manner the sheath 

 secures the waste products is by no means certain, although the 

 following explanation of the process may be advanced : — the 

 sheath-cell can, of course, absorb a considerable quantity of waste 

 products directly from the surrounding cœlomic fluid, which is 

 constantly set in motion by the contraction of muscles, etc., but 

 the cells can also obtain those materials from the blood corpuscles. 

 The latter having collected the waste products in the course of 

 their circulation are drawn to the sheath-cells, and there become 



