THS HABITS AND STRUCTURE OP ARENICOLA MARINA. 113 



extension of the blood-vessel supplying the funds of the nephridia. It 

 is not certain that there is a corresponding gonad on the first pair of 

 nephridia, but, on each of the following five pairs the gonads are present 

 during the breeding season. In both sexes the organ is a mass of cells, 

 from which the ova or spermatoblasts bi'eak away at a very early stage, 

 to ripen in the coelom. The rachis is continuous with the posterior 

 angle of the nephrostome, and is developed around a backwardly 

 projecting process of the nephridial vessel which comes off segmentally 

 from the ventral vessel (PI. IX., Fig. 18, G.V.). 



In large Arenicola, 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 development in the 

 genital rachis, the young reproductive cells may be found at the breed- 

 ing season in all stages of development in the ccelom. 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. X., 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 protoplasm. The production of ova by the fertile vas- 

 cular processes 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 spermotozoa in great 

 detail. The youngest stage which we have found in the coelom con- 

 tained eight spermatoblasts arranged round a vesicular-looking blasto- 

 phore (PI. X., 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 

 blastophore, together with a small quantity of a slightly fibrous 

 coagulum in the centre of the cavity. Curiously enough, perfectly ripe 



H 



