of the Fishery Board for Scotland. 



235 



D. — Some old mussels which had been living in an exposed situation 

 were obtained from St. Andrews on April 4, 1905, The shells were 

 thick, rough, dirty-coloured, and of a slatish hue. Some that were 

 opened were found to contain poorly-conditioned animals. According to 

 Fullarton, such mussels are called "Crocks" at Montrose; Scott and 

 Baxter say that they are in Lancashire known as " Blue-nebs." Several 

 of these, measuring 2\ inches long, were separated ; they were kept 

 in an aquarium or box with a current of water steadily flowing 

 through it. 



On April 6, one spawned ; a small pinkish mass was seen lying near it 

 on the bottom of the box. The eggs lay there till April 12. They were 

 then examined and found to be decomposing ; they were evidently not 

 fertilized. 



March 8, 1906. — One of the mussels was dead. 



May, 1906. — Another that was dead was measured and found to be 

 2 1 inches in length. 



August 5, 1906. — Two more dead shells measured each 2^ inches in 

 length. Two remain alive ; they measure 2^ and 2 T 9 ff inches. 



September 6, 1906. — One of the mussels has moved about on the 

 bottom of the box and left a track of byssi (see pp. 242 and 343). 



February 26, 1907. — The mussels measure 2^ and 2^ inches. 



March 22, 1907. — The mussels were put on to a floating tray, in 

 which they were submerged about 1 inch. 



April 13, 1907. — The mussels have made quite a heap of muddy 

 excreta below them on the bottom of the box. One had its round end 

 sticking up out of the water, and it remained thus for eight days, when it 

 sank again. 



June 11, 1907. — They measured (1) 2J inches, a male; it had a thin 

 mantle, and was getting ripe ; (2) 2f inches, rather thin mantle, was 

 getting ripe. 



In two years one has grown J inch ; the other has not increased in 

 size. 



Taken all over, the flesh of these mussels (A, B, C, and D) was in 

 average condition of fatness. Their shells, however, are characteristically 

 shaped. They are rounder, of greater thickness from side to side, but 

 not so high as more rapidly growing mussels. In figs. 26 and 27 are 

 shown two views of one of these mussels, and in figs. 31 and 32 two 

 views of one of the mussels of the same length, but which had grown 

 on a screen in the large pond. 



Conditions Favourable and Unfavourable to Growth. 



The rate of growth of the mussel depends on a number of factors 

 which may be briefly summarised as — (1) the quantity of food, (2) 

 salinity of the water, (3) presence and quantity of mud, sand, and fila- 

 mentous algae, (4) length of exposure to the air during each tide. 



Spatting. — As a rule mussels spat readily, and in large quantity ; 

 the scarcity of mussels in a locality is not usually due to a deficiency 

 of young. The young may not have a suitable area upon which to settle ; 

 they may settle too high up, and in consequence be uncovered for too 

 long a period each tide. They may settle on a high scaup and swamp 

 it, and the increased competition for food that results will militate 

 against the growth of the individuals. 



In July and August the surface of St. Andrews Bay, according to 

 MTntosh, swarmed with minute mussels of a somewhat circular 

 outline. The same author states that they appear in the zoophytes in 



