CRUSTACEA. 



being formed for progression on land, or, at least, for creeping at 

 the bottom of the sea. The tail, the great instrument of loco- 

 motion in the lobster, is here reduced to a rudiment, and the fin 

 at its extremity entirely obliterated ; the chela still continue 

 to be the most powerfully developed of the extremities ; while the 

 legs, the principal locomotive agents, are either terminated by 

 simple points, as in those species which are most decidedly ter- 

 restrial in their habits, or else, in the swimming crabs, the pos- 

 terior pair become expanded into flattened oars useful in nata- 

 tion (fig. 154). 



(365.) From the extreme hardness and unyielding character of 

 the tegumentary skeleton in Crustaceans, a person unacquainted 

 with the history of these animals would be at a loss to conceive the 

 manner in which their growth could be effected. In insects we 

 have seen that all increase of size occurs prior to the attainment 

 of the perfect condition, and expansion is provided for by the 

 moults or changes of" skin which take place during the develope- 

 ment of the larva ; but the Crustacean, having acquired its mature 

 form, still continues to grow, and that until it acquires in many 

 instances a size far larger than that which any insect is permitted 

 to arrive at. 



The plan adopted in the case before us, whereby growth is 

 permitted, is attended with many extraordinary phenomena. At 

 certain intervals the entire shell is cast off, leaving the body for 

 the time unfettered indeed as regards the capability of expansion, 

 but comparatively helpless and impotent, until such time as a new 

 shell becomes secreted by the dermis, and by hardening assumes 

 the form and efficiency of its predecessor. 



We are indebted to Reaumur,* who watched the process in 

 the Cray-fish (Astacusfluviatilis)^ for what little is known con- 

 cerning the mode in which the change of shell is effected. In the 

 animal above mentioned, towards the commencement of autumn, 

 the approaching moult is indicated by the retirement of the cray- 

 fish into some secluded position, where it remains for some time 

 without eating. While in this condition, the old shell becomes 

 gradually detached from the surface of the body, and a new and 

 soft cuticle is formed underneath it, accurately representing of 

 course all the parts of the old covering which is to be removed ; 

 but as yet little calcareous matter is deposited in the newly formed 

 integument. The creature now becomes violently agitated, and 



* Me"m. de la Acad. des Sciences, 1718. 



