186 



Consequently, the early crabs will probably cast eight or 

 nine times before the following summer, while the late 

 crabs may have cast only five times. There will, also, be 

 a corresponding difference in size. 



It will be realised, therefore, that it is quite 

 impossible to state the age of a crab with any degree of 

 certainty. The size of the crab not only depends upon 

 the frequency of casting, but also upon the ratio of 

 increase at each act of ecdysis. As we have already seen, 

 both these factors are subject to a considerable amount of 

 variation. The most valuable information on this point 

 is to be obtained from continued observations of crabs 

 kept in an aquarium. This has been done to a certain 

 extent by Mr. Waddington, of Bournemouth. But, even 

 in such cases, we are not justified in establishing any 

 broad principles on the results obtained. In the first 

 place, there is no doubt that captivity affects the frequency 

 of casting. There is also the same difficulty that obtains 

 amongst crabs living amongst natural conditions, viz., 

 that ratio and frequency of casting vary in individual 

 crabs. This is demonstrated quite clearly by an examina- 

 tion of AVaddington's : " three series, in which both factors 

 vary considerably. 



But in spite of the impossibility of forming any 

 definite laws with regard to the frequency of casting, a 

 careful examination of all the available facts and figures 

 enables one to give a general outline of the life-history of 

 Cancer with regard to this particular point. 



I propose, therefore, to give such an outline, but, in 

 doing so, I must emphasise what I have already said — 

 that we are dealing with factors which are by no means 

 constant. . 



I intend taking a purely hypothetical case, utilising, 



* Williamson. Twenty -second Annual Report, p. 135. 



