ON OUR PRESENT KNOWLEDGE Or THE CRUSTACEA. 55 



the smaller legs of the edible crab. The Eiver Crayfish (Astacus fluviatilis), tho 

 particular subject of the French naturalist's researches, as well as the common 

 lobster, havo such lines marked on the innor segment of their claw-legs as 

 leavo no doubt in my iniud that an opening takes place in them when tho 

 process of exuviation prococds, although not such a mechanical splitting as 

 Reaumur describes ; and it does not take place in any shape in the smaller, 

 or walking legs. 



" Tho same may bo said of the common Harbour-Crab (Carcinus mcenas), 

 Xantho fiorida, the Velvet-crab (Carcinus puber and pusiUus), and Atele- 

 cyclus heteroclon, all of which I have examined." 



It would be interesting to know how in tho male of the genus Gelassimus, 

 the enormously large distal joints of the prehensile claw can bo drawn 

 through the small opening of the coxa. I am much inclined to believe that 

 the splitting of the Avails at the point alluded to by Mr. Couch may bo more 

 for the purpose of enabling the animal to withdraw the great osseous tendon, 

 which at this joint must be extremely in the way of the passage of the rest 

 of the limb during the withdrawal from the old case. 



In a very large number of exuviae that I have examined I have never seen 

 the splitting process as described by M. Reaumur. 



Whatever may be the period, whether it be during the larva state or that 

 of the adult animal, the process of development under which the new shell 

 is produced must be similar. 



M.Milne-Edwards, in the introduction to his 'Histoire des Crustaces,' 

 p. 55, says that it is evidently secreted by the chorion, and moulds itself 

 upon the integument that covers it. 



In the ' Annals of Nat. Hist.' 1851, vol. vii. p. 298, are published some ob- 

 servations on the development of tho shell of crabs, with illustrations. I 

 state that I found immediately above the heart a mass consisting of nucleated 

 cells, areolar tissue (and blood-vessels?), extending to the internal surface of 

 the old shell, from which it is separated by a layer of pigment, which gives 

 colour to the new formation. Towards the base (that is, immediately above 

 the heart) the cells are uniformly large and distinct, while the areolar tissue 

 ramifies throughout tho whole. As advance is made from the base, colls of a 

 less size mix with them, which increase in number as they diminish in dia- 

 meter, until they approach the layer of pigment, immediately beneath which 

 they adapt themselves, by mutual pressure, into a polygonal form. Tho 

 layer extends over the whole periphery of the crab, immediately beneath the 

 shell, the thickness of the mass decreases with the distance from the centre, 

 and tho larger cells become fewer in number, the mass being chiefly made up 

 of tho smaller cells, which become lime-absorbing organs for the future shell, 

 which process commences previously to, and is completed after, the removal of 

 the exuviae. 



