St.- Andrews Marine Laboratory. 151 



sacs appeared to be undergoing degeneration. The females 

 at the end of May now contained comparatively few ova, and 

 the mantle had diminished considerably in thickness. Much 

 granular brownish pigment, moreover, was present in the latter. 

 The ova remaining in the mantle presented no structural 

 change. 



On the 9th June the female mussels still showed some ova 

 in the thickened dorsal region of the mantle, but the margins 

 were pale and thin. As before, much brownish pigment was 

 present, and those parts of the mantle devoid of ova were cellu- 

 lar or areolar. Throughout this month the sexual elements 

 steadily diminished. 



Finally, in July the dendritic appearance of the salmon- 

 coloured mantle was less marked, and neither ova nor sperma- 

 tozoa could be distinguished microscopically. The general 

 stroma was granular and minutely cellular, as if a general reso- 

 lution of the tissue were taking place ; and the characteristics 

 of the sexes were absent. 



The early stages in the development of the mussel had 

 therefore to be examined by aid of the tow-net on the surface 

 near the mussel-" beds " of the Eden, or in specimens kept in 

 sea-water in the laboratory (for at this time no pipes had been 

 fitted). 



On the 31st of May the keeper of the mussel-" beds" pro- 

 cured a number of minute mussels, about a line in length, from 

 the under surface of his boat, which had not been scraped 

 since last season. These had assumed the elongated outline 

 characteristic of the adult, and, in all probability, small as 

 they were, must have been the young of the previous season 

 or very early products of this. The ground for supposing 

 that they pertained to the previous season is the fact that the 

 surface of St. -Andrews Bay in July and August swarmed 

 with minute mussels (much less than the foregoing) of a 

 somewhat circular outline, that is, they had not yet assumed 

 the elongated outline above mentioned. They were much 

 younger than the forms procured by Lacaze-Duthiers* in the 

 Mediterranean, as they were settling on the blades of the sea- 

 weeds and similar structures within tide-marks, and which 

 showed four branchial processes behind the foot. At a later 

 stage they fix themselves by the byssus all round the margin 

 of the rocks, on wrecks, wooden stakes, crabs, and other 

 objects. 



The countless multitudes of these young forms in St.- 

 Andrews Bay show that there is a fine field for extending 



* Ann. des Sc. Nat. Zool. ser. iv. torn. v. (1856). 



