of the Fishery Board for Scotland. 



219 



' (February) advanced, the mantle, especially in the neighbourhood of the 

 ' organ of Bojanus, increased considerably in thickness, so that the region 



* hung downward like a pouch or flap on opening the valves. The whole 



* surface of the mantle becomes speckled in both sexes with the reproduc- 

 ' tive elements. On puncturing the enlarged region of the organ of 

 ' Bojanus in a male on the 17th February, a milky fluid composed of 



* sperms exuded. The mantle in this case had increased in thickness about 

 ' f of an inch, and the development of the sperm-sacs seemed to proceed 

 ' from the dorsal to the ventral edge of the mantle.' Professor M'Intosh 

 found that in general the mussels reached full reproductive maturity in 

 April ; thereafter the ova and spermatozoa gradually disappeared from the 

 mantle, until in July those he examined were spent. In July and August 

 he noticed that the surface of St Andrews Bay ' swarmed with minute 

 ' mussels of a somewhat circular outline,' and ' they were much younger 

 ' than the forms procured by Lacaze-Duthiers in the Mediterranean, as they 



* were settling on the blades and similar structures within tide marks, 

 ' and which showed four branchial processes behind the foot.' The above 

 is a brief resume of the literature of the special subject in hand. There is 

 an immense amount of published information relating to mussel-culture, 

 such as statistical accounts of the number of mussels sold or used for bait, 

 discourses on the comparative efficiency of the French and British methods 

 of culture, practical suggestions for the improvement of existing beds and 

 the acquisition of new ones, and the like. No systematic effort seems to 

 have been made to understand the early life-history of the mussel, and it 

 was with the view of unravelling this that, at the request and under th^ 

 guidance of Professor M'Intosh, the writer made a series of observations 

 during the summer and early autumn of this year (1885). 



If a mussel in a matured condition is cut from its shell, the whole 

 outer envelope or mantle is seen to be filled with reproductive elements. 

 Besides the mantle there is a central wedge-shaped mass behind the foot 

 — the * abdomen * — which is also filled with them. This organ is con- 

 nected with the mantle in the pericardial region. The outer wall of the 

 liver (a dark-coloured organ anterior to the foot) is usually devoid of the 

 dendritic sperm-sacs or ovaries, but in an occasional example they are seen 

 to ramify there to a small extent, as outgrowths of the abdominal mass. 

 This is a condition in striking contrast with what obtains in the horse 

 mussel {Mytilus modiolus), a much larger species inhabiting deep water. 

 In it there is no trace of reproductive follicles in the mantle, but these 

 proceed forward from the greatly enlarged central mass, and in many cases 

 almost completely hide the liver. In both species of mussels the female 

 reproductive organs are almost invariably of a deeper red or orange tinge 

 than those of the male. (Half -sized specimens of both sexes may be 

 found with the organs pale). Methylated spirit immediately extracts 

 the colouring matter. A close inspection enables one to distinguish the 

 sexes by the naked eye, the sperm -sacs being mostly arranged in more 

 prominent groups than the ovigerous masses. There is probably no means 

 of discerning the difference by any feature in the exterior of the shell. 

 The spermatozoa of the common mussel have a characteristic shape, differ- 

 ing distinctly from those of the horse mussel. The former have a balloon- 

 shaped head, tapering continuously off to a fine tail, whereas the head of 

 the latter has a constriction where it tapers off to the tail. When an 

 incisioD is made in the fully-ripe male organ a creamy fluid issues, con- 

 taining immense numbers of spermatozoa, the motion of which is well de- 

 scribed as ' dancing.' The milkiness is in proportion to the ripeness of 

 the specimen. If the spermatozoa are in packets or groups, they are pro- 

 bably unripe. When the ovarian tissue is cut into, the fluid therefrom 



