184 



Rate of Growth. 



From the evidence obtained by Williamson and 

 Waddington, and also from observations taken in the 

 course of the present work, it would appear that the rate 

 of growth varies considerably. For example, in a series 

 of seventeen crabs measured in Heligoland I found that 

 the fraction of increase varied from ^ to -^ (see table 

 on p. 382). In Waddington's series published by 

 Williamson* the fraction varied from i to --, and in the 



o 5 



table published by Williamsont the ratio of increase 

 varies from ^ to \. But, speaking generally, it may be 

 said that this ratio is between \ and \. 



It is not surprising that this rate of growth should 

 be a variable one. The main factors to be considered are 

 probably the general health of the animal (i.e. the 

 condition of the tissues) the amount of food, the purity of 

 the water and the temperature of the water, and many 

 other causes. Any one of these factors would alone be 

 capable of affecting very considerably the rate of growth. 



I give a table (p. 382) showing the rate of increase in 

 seventeen different crabs. These measurements were 

 taken from specimens in the JSTordsee Museum, Heligo- 

 land, by kind permission of the Director, Professor 

 Heincke. 



Frequency of casting. 



Speaking generally, it may be said that in the 

 younger stages the moultings are frequent, but that as 

 the crab grows older the period between each process of 

 ecdysis and the next becomes longer. 



In the earliest stages the frequency of casting differs in 



* Williamson. Twenty-second Annual Report, Fishery Board for 

 Scotland, p. 135. 



t Williamson. Eighteenth Annual Report, Fishery Board for 

 Scotland, p. 110 (see also Twenty-second Report, p. 122). 



