ANATOMY AND CLASSIFICATION OF THE ARENIOOLID^']. 521 



granules, though not quite in such large numbers as those of 

 the middle portion of the nephridium. The whole nephridium 

 is covered by a very thin layer of coelomic epithelium. In 

 both this and another post-larva we have carefully examined 

 the segments anterior and posterior to those in which the 

 nephridia occur, but have found no traces of rudimentary 

 nephridia in these segments. 



14. Reproductive Organs. 



The reproductive organs of A. marina have been described 

 by Cosmovici (1880), Cunningham (1887, 1888), Kyle (1896), 

 and others, but until very recently these organs had been 

 investigated in this species only. Our former paper (1898, 

 p. 33) contains the first reference to the large and complicated 

 gonads of A. e can data, and a brief description of them has 

 been subsequently given by Fauvel (1899). 



The reproductive organs of Arenicola are closely asso- 

 ciated with the nephridia. Immediately behind and con- 

 tinuous with, the posterior portion of the nephrostome there 

 is a small whitish or pinkish mass of cells, ovoid, club-shaped, 

 or cylindrical (PI. 24, fig. 29, and PL 26, figs. 53, 54). This is 

 one of the reproductive organs, ovaries or testes as the case 

 may be. The gonad is developed around, or in contact with, 

 a large branch of the afferent nephridial vessel, which we may 

 call the gonidial vessel. The afferent nephridial vessel, after 

 giving off a branch to the seta8 and body-wall, generally gives 

 off a branch to the secretory portion of the nephridium, the 

 main portion of the vessel traversing the dorsal lip of the 

 nephrostome. On entering the nephrostome, this vessel often 

 gives off a branch which runs across the ventral lip, and joins 

 the vessel from the dorsal lip again as it leaves the nephro- 

 stome (PI. 26, figs. 46, 48). The vessel which leaves the 

 nephrostome and runs down the inner side of the secretory 

 portion of the nephridium is the gonidial vessel, and around 

 this the gonad is developed by proliferation of its peritoneal 

 covering. 



