STRUCTURE OF THE NEPHRIDIA OF DINOPHILUS. 519 



consecutive series of changes in the form of the nephridia 

 from the more primitive flame-cells of the Turbellaria to the 

 complicated organs of Polychaets, and so establish a definite 

 connection between the nephridia of these two groups, which 

 at present would seem to be separated distinctly from one 

 another. 



Although I made a careful search for Dinophilus at 

 Plymouth during the latter part of June, 1903, the season 

 was too late, and the adults had already disappeared from 

 their usual haunts. Until the present season I have had no 

 opportunity of revisiting Plymouth. This year, however, no 

 difficulty was experienced in obtaining an abundant supply 

 of material, although limited to the male form. Towards 

 the latter part of April I examined a considerable quantity of 

 Dinophilus material daily without observing a single 

 female. This apparent absence of the female I am unable 

 to account for, except on the ground that the breeding 

 season may have been passed, as young worms of all sizes 

 were observed in numbers, and the males seemed to have 

 already discharged their spermatozoa.^ Both sexes disap- 

 pear entirely at Plymouth, I believe, towards the end of May 

 or the fu'st part of June. During July and August they are 

 not to be found at Plymouth. 



This disappearance of Dinophilus during certain months 

 has already been noticed by Hallez (7), Weldon (24), and 

 Harmer (8). Weldon (24) observed the adults of D. gigas 

 in April undergoing degeneration after the discharge of the 

 sexual products, and concluded that the worms periodically 

 died oft" after the breeding season. That this can hardly be 

 the case has been shown by Schimkewitsch (23) in D. vorti- 

 coides, where the females continue to live long after they 

 have laid their eggs, and in fact pass through several breed- 

 ing periods during the course of the year. In an American 

 species Moore (17) has found that the worms have the power 

 of forming capsules and of encysting themselves. On being 

 ^ Harmer (8) states that about April 18tli the females were still carrying 

 their eggs. 



