NBPHRIDIA. OF THE AOTINOT ROCHA LARVA. 113 



cells/' remains the best yet published. Caldwell (2) added 

 greatly to our knowledge of the structure and development 

 of these organs. '' The nephridium/' he writes^ " is a ciliated 

 canal with cellular walls. The canal is not formed of per- 

 forated 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 mesoblast" (the lining of the 

 trunk coelom) . " Attached to its inner end/' he adds, " are 

 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 processes, 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-swim- 

 ming life of [the] larva does the excretory canal system 

 open into the body-cavity." 



None of the authors who succeeded Caldwell have given 

 such an accurate account of the structure of the nephridium. 

 De Selys Longchamps merely confirms his statement that it 

 does not open into the pre-septal hgemocoel. Ikeda (9) comes 

 to the same conclusion, and describes " spindle-shaped excre- 

 tory cells" on the blind internal extremity, and an epithelium 

 covering the nephridial tube. 



Masterman, on the other hand, led astray by the desire to 

 compare the nephridium of Actinotrocha with that of Amphi- 

 oxus as described by Boveri, considers that it opens by several 

 funnels into the pre-septal cavity, which he mistook for 

 coelom. As pointed out by Ikeda, Masterman confused the 

 mass of blood-corpuscles with the " excretory cells " of 

 Caldwell : " a pair of large bodies, apparently aggregations 

 of amoeboid cells, with large nuclei The cells are 



VOL. 47, PAKT 1. NEW SKKIKS. H 



