POLYCH^TA SEDENTARIA OF THE FIRTH OF FORTH. 651 



undifferentiated that it was not possible at first to be certain it was a gonad. 

 But in specimens which contained but few ova in the ccelom, it was obvious 

 enough that this mass of cells was the female gonad, some of the cells in it 

 being distinctly recognisable as ova (fig. 13 d). The ova in the body-cavity 

 were of various sizes (fig 13 e), and it was evident that they went through the 

 greater part of their development after being detached from the gonad. Male 

 elements were in other specimens found freely suspended in the liquid of the 

 ccelom, but only in small quantity ; they were in the form of sperm polyplasts, or 

 bundles of spermatozoa (fig. 13 f). The gonad in this case showed, instead of 

 ova and germinal cells, systems of small cells, 6 or 8 in each system, evidently 

 derived from the segmentation of germinal cells, and destined to fall off into the 

 ccelom, and there form sperm-polyplasts, and ultimately spermatozoa. 



None of the worms examined in February and March were sexually 

 mature, and it was afterwards found that Arenicola marina does not shed its 

 genital elements till August and September. Max Schultze was, we think, 

 mistaken in ascribing the ova and embryos whose development he describes 

 (Entw. von Arenicola, etc., Halle, 1856) to this species. As shown in the following 

 section, the cocoons he gathered were most probably these of Scoloplos 

 armiger. It is yet uncertain whether the present species forms cocoons at all, 

 or sheds its ova separately in the mud. 



Eggs and Larvoe belonging, according to Max Schultze, to Arenicola. 



At the beginning of February, we found large numbers of gelatinous cocoons 

 on the surface of the sands near the Station at low tide. From Max 

 Schultze's description (loc. cit.) we concluded at first that these were the 

 spawn of Arenicola marina. That author discovered the cocoons he described 

 as belonging to Arenicola on the flat shores of the island of Neuwerk, not far 

 from Cuxhaven. In most points his account suits the spawn we obtained, but 

 his cocoons had a pink colour, due to the ova within. We have not observed 

 any colour in our specimens ; the jelly was transparent, and the ova and em- 

 bryos opaque white. The colour may not be a constant attribute. 



The cocoons or gelatinous masses are about 2 cm. long and 1 cm. broad. 

 They are pear-shaped, and the narrower end is produced into a long cylindrical 

 stalk, about 3 cm. in length, which contains no ova. The stalk is usually 

 imbedded in the sand. 



It was very easy to keep these ova alive, and hatch them in captivity. They 

 were simply placed in a shallow dish with sea water, and a little sand at the 

 bottom. The larvae lived several days after hatching. PI. XL. figs. 14 a to 14 f, 

 show a series of stages in the development of these larvae. There are three 

 transverse bands of cilia, which persist until the setae begin to appear, and there 



VOL. XXXIII. PART III. 5 C 



