492 CreBßwell Shearer 



give a description of these structures, as well as to follovv in part, 

 the developraeut of the nephridia. His account remaining for mauy 

 years tlie only one we possessed, it has become well known from 

 text-books. Attacbed to the euds of the nephridial eanals Caldwell 

 described a number of cells of peculiar form. "Each cell has a 

 nucleus and a process similar to those of the ordinary mesoblast 

 cells. By one of these the cell is attached to the end of the large 

 canal. This process is larger than the fine process and has a 

 cylindrical form. By the canal formed inside the cylinder, small 

 brown concretions seen in the cell itself pass into the larger canal 

 and so to the exterior. ... At no time during the free-swimming life 

 of the larva does the excretory canal system open into the body- 

 cavity" (pag. 376). Regarding the origin of the nephridia, Cald- 

 well (1) in a paper published subsequently to the one just quoted, 

 thought they arose in connection with the diverticula into which the 

 anal pit divides. These open on the exterior in the middle line. 

 "The closure of this openiug proceeds in such a way that each })Ouch 

 remains open to the exterior by a small pore on either side. ... I 

 believe — that each pore persists as the opening of the nephridium 

 of its own side." The formation of the excretory cells which lie in 

 the blastocoel and not in the body-cavity, — "I bave independently 

 traced from the mesodermic cells of the posterior pouches". Finally 

 Caldwell quoted Hatschek as believing that the whole organ arose. 

 from the mesoblast (pag. 19). 



Ikeda (14) was the next to give a detailed description of the 

 nephridia. He also ascribed their origin to Caldwell's posterior 

 pouches. The anal pit sinking in from the ectodermal surface forma 

 two pouches which in time give rise to two blind tubes which 

 project into the preseptal haeniocoel, forraing the nephridial canals; 

 a certain number of mcsenchyme cells attach themselves to the ends 

 of these tubes and become later the solenocytes. He placcs these 

 organs under the heading of mesoblast, although the nephridial canals 

 are of ectodermal origin, "the organs as a whole bear intimate 

 relations to the mesoblast". Of some interest is the discovery by 

 Ikeda of a pair of retractor musclcs running down from the Oesophagus 

 to the body wall in the region of the nephridia, which resenible 

 the retractor muscles of the Trochophore larva. 



De Selys Longchamps (24, 25) has confirmed many of the 

 points in Caldwell's and Ikeda's description of the nephridia. 

 First, that they never open into the body cavity, and secoudly, 



