INTRODUCTION. XXXV 



crust may be perceived, upon pressing it gently in dif- 

 ferent parts. Shortly afterwards, — and this description 

 belongs particularly to the river cray-fish, — it appears 

 uneasy and restless, rubbing its limbs against each other, 

 and moving the segments of the body in various direc- 

 tions. It throws itself on its back, and, swelling out its 

 body, ruptures the membrane which connects the cara- 

 pace with the abdomen, and raises the former, so as to 

 loosen it from its attachments. Resting from time to 

 time after its laborious efforts, it finally detaches the 

 whole thoraco-abdominal portion, from which it withdraws 

 itself, having, with much apparent difficulty and pain, 

 disengaged the legs, and then the antennre, the eyes, and 

 other appendages. It is impossible to imagine that the 

 crust of the legs, and especially of the great claws of the 

 larger species, could be cast off unless it were susceptible 

 of being longitudinally split; and Reaumur states that 

 such is actually the case; each of the segments being 

 composed of two longitudinal pieces, which, after sepa- 

 rating to allow of the passage of the soft limb, close again 

 so accurately that it is very difficult, in the cast crust, 

 to discover the line of division. When the animal has 

 disembarrassed itself of the crust, the latter is found abso- 

 lutely entire, and has exactly the form which it possessed 

 previous to the operation. In a recent interesting ac- 

 count of the exuviation of a Maia,* Mr. Gosse has, how- 

 ever, shewn that in this brachyurous form, no such split- 

 ting of the legs takes place, but that " the animal pulled 

 first at one and then at another, until they were quite out, 

 as if from boots. The joints as they came out were a great 

 deal larger than the cases from which they proceeded. It 



* Annals of Nat. Hist. 2nd Ser. vol. x. p. 210. 



