ANATOMY AND CLASSIFICATION OF THE ARENICOTJD^. 527 



round the nucleus^ the peripheral portion of the protoplasm 

 containing very little yolk (fig. 51). 



The ripe ova of the various species are almost equal in 

 size. Those of A. marina are the largest. They are gener- 

 ally spherical, about '16 mm. in diameter, and have a distinct 

 but thin vitelline membrane. The ripe ova of A. Claparedii 

 are spherical, or sometimes rather oval, about .'14 mm. in 

 diameter, and similar in every respect to those of the common 

 lug worm. The spherical or slightly oval ova of A. cristata, 

 which are about '14 mm, in diametei', have a rather thick 

 vitelline membrane, thus differing from the two species pre- 

 viously mentioned. The ripe ova of A. Grubii and A. 

 ecaudata are identical in shape, size, and other characters. 

 They are generally oval in shape, though some are spherical. 

 The average length of their longer diameter is '13 mm. to 

 •14 mm. They have a thick vitelline membrane (fig. 51). 



The spermatozoa of all species of Arenicola develop in a 

 manner similar to those of A. marina, briefly described in 

 our former paper (1898, p. 32, and PI. 5, figs. 29 — 34). 

 Except in A. ecaudata the male genital cell falls from the 

 testes after two or three divisions, and completes its develop- 

 ment while in the coelomic fluid. Farther divisions result in 

 the formation of a disc-shaped mass of spermatids containing 

 a central cavity in which there is a small coagulum, the 

 remains of the blastophore. We have hitherto seen only the 

 ripe spermatozoa of A. marina (1898, PL 5, fig. 29), but 

 those of A. Grrubii have been seen and figured by Claparede 

 (1868, p. 299, and PI. 19, fig. 2, c). The latter closely 

 resemble those of A. marina, except that they are somewhat 

 smaller. 



The genital products escape by means of the nephridia. 

 Although the first pair of nephridia do not bear gonads they 

 nevertheless, like the other nephridia, serve as genital ducts. 

 The terminal portions of the nephridia are, during the breed- 

 ing season, often distended with ova or spermatozoa. In a 

 specimen of A. marina, 200 mm. long, the vesicles of some 

 of the nephi'idia measured 14 mm. in length and about 



