392 CEUSTACEA. 



their development. But the most singularly altered portion of the 

 skeleton is the fin of the tail, which here becomes transformed into a 

 kind of holding apparatus, by which the creature retains a firm grasp 

 upon the bottom of its residence. 



(1001.) In the Brachyura, or Crabs, we have at once, in the concen- 

 tration observable in all parts of the skeleton, an indication of its being 

 formed for progression on land, or at least for creeping at the bottom 

 of the sea. The tail, the great instrument of locomotion in the Lobster, 

 is here reduced to a rudiment, and the fin at its extremity entirely 

 obliterated ; the chelae 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 terrestrial in their habits, or else, in the Swimming Crabs, the 

 posterior pair become expanded into flattened oars useful in natation 

 (fig. 200). 



(1002.) 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 development 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. 



(1003.) 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 unfet- 

 tered 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. 



(1004.) "We are indebted to Eeaumur*, who watched the process in 

 the Cray-fish (Astacus fluviatilis) , for the first account of the mode in 

 which this change of shell is effected. In the animal above mentioned, 

 towards the commencement of autumn, the approaching moult is indi- 

 cated by the retirement of the Cray-fish into some secluded position, 

 where it remains for some time without eating. While in this condi- 

 tion, 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, 

 by various contortions of its body, seems to be employed in loosening 

 * M&n. de 1'Acad. des Sciences, 1718. 



