376 



Mr. W. H. Caldwell. On the 



[Dec. 7, 



the alimentary tract. The cells of the first stomach, however, though 

 ciliated, are much more amoeboid than in the adult. Throughout 

 larval life intra-cellular digestion goes on in this region. This mode 

 of digestion ceases with the metamorphosis. 



With the formation of the anus this end of the body gradually 

 grows out. The papilla with the anus at its end enlarges, and finally 

 forms the largest part of the full-grown larva. Tentacles appear in 

 pairs as outgrowths along the lines of the postoral ciliated ring, new 

 tentacles appearing dorsaliy. 



The further development of mesoblast proceeds always in continuity 

 with the first pair of lateral diverticula. The body-cavity of the hind 

 end of the larva is formed independently in a paired mass of cells 

 which grows out from the end of the first formed sacs, and remains 

 separated from the latter by a septum. 



Thus the whole- mesoblast of the animal arises as two endodermic sacs, 

 the walls of which form somatic and splanchnic layers. 



Nephridia. 



On either side of the body lies a ciliated canal with cellular walls. 

 This canal is not formed of perforated cells. 



Each canal opens to the exterior behind the septum on either side 

 of the opening of the foot. The canal lies outside the somatic meso- 

 blast. 



Attached to its inner blind end are a number of cells of very 

 peculiar form. Each cell has a nucleus and processes similar to those 

 of ordinary mesoblast cells. By one of these the cell is attached to 

 the end of the large canal. This process is larger than the free pro- 

 cesses, and has a cylindrical form. By the canal formed inside the 

 cylinder, small brown concretions seen in the cell itself pass into the 

 large canal, and so to the exterior. These excretory cells, with their 

 fine canals, increase in number with the growth of the larva. They 

 float freely in the body-cavity in front of the septum. 



The cells are similar to the perforated cells which form the internal 

 ends of the nephridia described by Hatschek in Echiurus.*' 



At no time during the free swimming life of larva does the excre- 

 tory canal system open into the body-cavity. 



With regard to the development of the nephridia I have observed 

 somatic mesoblast cells, at the time when the mesoblastic sacs of the 

 trunk are forming, take the characteristic shape of excretory cells with 

 cylindrical processes. 



On the other hand, I have failed to discover the origin of the main 

 ciliated canal. 



* Berthold Hatschek, " Ueber Entwiek. von Echiurus," Arbeit, a. d. Zool. Instit. 

 Wien, vol. hi, 1880. 



