38 



valves assuming an oval form, the foot developmg, the 

 posterior adductor muscle present, rudiments of four gill 

 filaments appearing, and the velum still functional is, at 

 the outside, a month old. Subsequently to this (0'28 mm. 

 long) blue pigment is deposited round the margin of the 

 shell, the foot becomes the organ of locomotion, and the 

 larva prepares for fixation by the development of byssus 

 gland and byssus. 



Free-swimming larvae, somewhat younger than the stage 

 last referred to, that is, about 0'25 mm, in diameter, and 

 provided with circular valves showing no trace of pigment, 

 were taken by Mr. Scott in a tow-net gathering at 

 Piel, on September 9th, last year. These Mussels were 

 certainly less than one month old, and their appearance at 

 this time is in accordance with anatomical observations 

 made on specimens taken in the neighbourhood, which led 

 us to fix July and early August as the months during 

 which spawnmg took place. But the evidence given by 

 other observations of this kind is very perplexing. Thus 

 young Mussels with circular or oval valves ranging in size 

 from 0'27 to 045 mm., and with the rudiments of four or 

 more gill filaments present, were taken in June, 1892, 

 fixed to various zoophytes, and assuming the age of these 

 not to exceed a month, the time of spawning is thrown 

 back to May or early June. An examination of the 

 in-shore tow-nettings shewed that young Mussels of 

 approximately this size (0"3 to 0'6 mm.) were taken in the 

 estuary of the Bibble on January 27th, 1898, and others, 

 varying in size from 02 to 0'7 mm., were found in Ulver- 

 ston Channel on February 4th. Some of these had 

 probably been already fixed and were loosened by the 

 force of the tide, but others had all the characters of the 

 fi'ee-swimming stages described above, and their presence 

 at this time is only explicable on the assumption that 



