ON THE DEVELOPMENT OF LINGULA ANATINA. 61 



The nephridium is very conspicuous in the advanced larva? 

 (7-15 p. c. stage). But at the 5-6 p. c. stage it is quite invisible 

 in life from without, and its presence is only detected in stained 

 toto preparations or in sections. I shall first describe the organ 

 in the 5-6 p. c. stage and then in the 7-9 p. c. stage. 



On a ventral view of the larvœ at the 5-6 p. c. stage we see 

 a pair of cell strands (PI. VI., Fig. 82, nph.) whose posterior 

 part forms the ileo-parietal band. In transverse sections of the 

 larvae of the same stage we see at the level of the M. occlusores 

 anteriores a pair of small protuberanes on the lateral body walls 

 near the ventral shell valve (PL VII., Figs. 92, 108, nph.). 

 From the protuberance backward there is found in the body wall 

 a cell rod, which pushes aside the peritoneal epithelium from 

 the epidermis. The rod is composed of highly glandular cells. 

 Somewhat later a narrow lumen appears in it, its cells coming 

 now to be so arranged as to form a tube, which in a transverse 

 section is seen to consist of two or three cells. Judging from 

 the length attained at the present stage we can infer that the 

 organ had made its appearance at the 4-5 p. c. stage. Lack of 

 material at this stage, however, prevented me from ascertaining 

 the exact origin of the organ. 



At the next stage (7-9 p. c. stage) the nephridium becomes 

 so conspicuous that we can easily detect it in fresh specimens. 

 In an examination of toto preparations we find that the differention 

 of the nephridial tube into the tube-part and the funnel has 

 already taken place (PI. VI., Fig. 85, and PI. VIL, Fig. 107). 

 Thus, little by little, the orginal tube becomes converted into a 

 structure which can easily be reduced to the conditions of the 

 adult nephridium. Fig. 109 (PL VII.) shows a longitudinal 

 section through the axis of a larval nephridium. We see that in 



