HABITS AND STRUCTURE OF ARENICOLA MARINA. 31 



nephridial vessel which comes oflF segraentally from the ventral 

 vessel (PI. 4, fig. 18, G. V.). 



In large Areuicola, at certain seasons, the vascular process 

 has no gonad, and it is possible, as Cuenot (1891) suggests, 

 that a formation of the amoeboid corpuscles of the coelom takes 

 place at this point when the animal is not breeding. 



After passing through the earliest stages of their develop- 

 ment in the genital rachis, the young reproductive cells may 

 be found at the breeding season in all stages of development 

 in the coelom. The ova do not exhibit any considerable changes 

 except in size in attaining maturity. They are nourished 

 either directly from the ccelomic fluid, or possibly (Cuenot, 

 1891) by the amoeboid cells acting as follicle-cells, though 

 we have seen nothing to support this view. Extrusion of a 

 polar cell (?) has been observed by us in an ovum only about 

 half the definitive size (PI. 5, fig. 35, a and b). In the spherical 

 ripe ova (which measure "16 mm. in diameter) a distinct but 

 very thin vitelline membrane is present, and a small quantity 

 of food-yolk in the form of very small granules in the proto- 

 plasm. The production of ova by the fertile vascular pro- 

 cesses of the nephrostomes must be extraordinarily great, 

 since the spacious body-cavity of a large worm is eventually 

 filled to bursting with them by the end of February. 



We have not followed the development of the spermatozoa 

 in great detail. The youngest stage which we have found in 

 the coelom contained eight spermatoblasts arranged round a 

 vesicular-looking blastophore (PI. 5, fig. 30). Further division 

 and elongation of the outer ends of the cells to form the tails 

 of the spermatozoa produces the stages seen in figs. 31 to 34. 

 The masses of spermatids are not spherical, but disc-shaped, 

 their thickness being only about one quarter of their long 

 diameter. They contain a cavity, the remains of the blasto- 

 phore, together with a small quantity of a slightly fibrous 

 coagulum in the centre of the cavity. Curiously enough, 

 perfectly ripe males were comparatively rare in March and May 

 of this year, when mature females were abundant. In most 

 cases the body-cavity was full of spermatids in great bundles, 



