37 



spermatozoa from the time when these have accumulated 

 in considerable quantity in the gonads, that is to say, from 

 the beginning of April on to the beginning of the summer 

 spawning period. And it seems certain, considering the 

 variability observed in the ripening of the gonads, that 

 isolated individuals may undergo complete spawning a 

 considerable time in advance of, or later than, the date of 

 occurrence of the maximum. However that may be, there 

 can be no doubt that the number of larvae resulting from 

 spawning in the early part of the year bears a very small 

 proportion to those produced during the maximum spawn- 

 ing period in the summer months. 



There is still some uncertainty regarding the rate of 

 growth of the young Mussel, and this is due probably to 

 variations contingent on the conditions under which the 

 adult animal spawns, and the larva undergoes its early 

 development. Most probably in the early stages during 

 which the young Mussel has a free-swimming existence, 

 the development and rate of growth are fairly constant, 

 but with the acquisition of the byssus and the fixation of 

 the larva, the subsequent growth is dependent, to a large 

 extent, on the situation it finds itself in, the supply of food, 

 the extent to which the larvae are crowded together, and 

 on the time of year in which spawning of the parent 

 occurred. From the observations made by Wilson,* who 

 succeeded in artificially fertilizing eggs of the Mussel, and 

 tracing out the early development, it appears that the 

 larva, about 0*15 mm. in length, provided with semicircular 

 valves, showing the first rudiment of the anterior adductor 

 muscle, and still using the velum as a locomotive organ, is 

 at most 12 days old. An older stage than this, with the 



•J. Wilson. — "On the Develoiiment of the Common Mussel." Annua 

 Reiwrtof the Fishery Board for Scotland for the year 1885, pp. 218—222, and 

 Report for 1886, pp. 247—256, Plates XII.— XIV. 



