522 P. W. GAMBLE AND J. H. ASHWORTH. 



Gonads are not present on the first pair of nephridia^ but 

 tlie gonidial vessel has the usual position and relations. On 

 the first uephridia of A. e can data (as will be further 

 described below) there is, however, a distinct strand of tissue 

 homologous with the genital strand, and occupying a similar 

 position, but not producing reproductive cells (PI. 26, figs. 

 46, 48, 0. 8t7\). The gonad is a closely packed mass of cells, 

 in which at the anterior end, i. e. at the end in contact with 

 the nephrostome, the cells are small, almost uniform in size, 

 and have well-marked, deeply staining nuclei. In the middle 

 and posterior portions of its length, the cells on the surface of 

 the gonad become differentiated, and in females young ova, 

 in males young spermagonia, may be recognised. The 

 anterior portion of the gonad is covered by a thin layer of 

 coelomic epithelium, but the portion of the gonad from which 

 young ova or spermagonia are being shed is not covered by 

 an epithelium. The ova remain attached to the gonad until 

 they have attained a diameter of 16 /x to 20 ju. On reaching 

 this size they are shed from the surface of the ovary into the 

 coelom, where they complete their development. At the sur- 

 face of the testes there may often be seen groups of two, four, 

 or eight cells — young spermagonia, — which at this stage are 

 shed into the coelom and there undergo the numerous 

 divisions resulting in the formation of the large masses of 

 spermatids figured in our former paper (1898, PI. 5, fig. 34). 

 These remarks on gonads apply to all the species except 

 A. e can data, in which the reproductive organs are much 

 more complicated (see p. 524). 



The gonidial vessel is developed on the nephridia in very 

 early life. It may easily be recognised in a specimen of A. 

 marina 17 mm. long; and in the last four nephridia of a 

 specimen of A. Grubii 35 mm. long, the cells of the peritoneal 

 covering of the gonidial vessel have already begun to pro- 

 liferate, and have given rise, for a distance of "5 mm. to "6 mm. 

 to a cylindrical covering of small uniform reproductive cells 

 with well-marked nuclei. In A. marina the gonads do not 

 appear until the specimens are somewhat larger ; e.g. they 



