ON OUK ritESENT KNOWLEDGE OP THE CRUSTACEA. 49 



Milne-Edwards contends that this splitting of the carapace in the Brachyura 

 is a necessity demanded by the formation of the animal ; and he says that 

 in those Brachyura where this suture does not exist he is inclined to believe 

 that exuviation takes place as described by Reaumur in the crayfish. Ono 

 argument against this idea is that this suture exists in every one of the Bra- 

 chyurous forms of Crustacea. 



The new tegumentary structure continues in a soft condition, according to 

 Collinson, for a longer period than described by Re'aumur in the Crayfish ; and 

 the period of moulting for these animals is considered a period of sickness. 

 They generally during this state hide themselves in sheltered places where 

 they may be best protected from the animals to which they are most liable to 

 become a prey. Some hide themselves beneath stones, others burrow into 

 the mud. At this period some exotic land-crabs are most preferred for their 

 edible properties ; but the marine forms aro valueless for food. 



In tho ' Annals of Nat. Hist.' vol. vii. p. 29S, 1851, the author of this 

 Report gave an account of the manner in which he had observed exuviation 

 take place in the common Shore-Crab ( Oarcinus mcenas). 



The manner in whicli it appears to free itself from the skin depends upon 

 tho internal growth of the animal. 



From the period of quitting the ovum to that of old age the skin is thrown 

 off at certain periods. When very young it is accomplished every few days ; 

 as the animal grows larger, weeks and then months interrene, until the 

 animal arrives at an adult condition, when it is cast but once a year ; and 

 when it has become old, and ceases to increase in size, it is probable that the 

 shell is cast off less frequently, if we are to judge from tho state in which 

 specimens have been taken on which oysters of two or three years' growth 

 have been found attached to the animal. In old age the absence of the 

 internal growth appears to be wanting as a stimulus for the reproduction 

 of the new skin. 



Tho increased bulk of the growing animal becomes compressed within 

 limits too small. The old carapace is therefore raised out of its position bv 

 the mechanical pressure of the internal structure ; and one of the first signs of 

 the approaching change in the animal's economy is an increased thickness of 

 the animal. 



As the period of exuviation approaches the crab wanders about in search 

 of a retired spot, and frequently exhibits a savage disposition, darting at any 

 thing that approaches it. When it finds a suitable position, it inserts the 

 point of one or more of its legs into some crack or crevice, and withdraws 

 itself from tho old skin by raising the carapace, and escaping between it and 

 the pleon, in the manner described by Re'aumur and Collinson. 



The carapace of the new structure is at first in a very wrinkled and 

 crumpled condition ; but it almost immediately expands to its full size, thus 

 becoming much larger than its old proportions, and continues without further 

 increase in dimensions until its next period of exuviation. 



The animal has the power for a certain period of retaining its shell at will 

 until suitable circumstances both as to time and place occur for the casting 

 of its shell with security. It appears also to be very shy. In several 

 instances I have seen animals before and after the process has commenced, 

 having patiently watched for hours at a time without success to seethe actual 

 process proceed ; upon returning, after a few minutes' absence, I have found 

 the old skins cast off, and the animal at rest by the side of it. This would 

 indicate that it is done rapidly, if not with ease. 



1877. b 



