CRUSTACEA. 



whose opportunities enable them to prosecute researches connected 

 with their elucidation.* 



(366.) The structure of the articulations which unite the differ- 

 ent segments of the skeletons of the Articulata, and the general ar- 

 rangement of their muscular system, have already been described ; 

 and, in the class before us, these parts of their economy offer no 

 peculiarities worthy of special notice. 



(367.) Throughout all the Crustacean families the alimentary 

 canal exhibits great simplicity of arrangement, and consists of a short 

 but capacious resophagus, a stomachal dilatation or cavity in which 

 is contained a singular masticatory apparatus, and a straight and 



* Since writing the above, I have been fortunate in procuring a very good specimen 

 of Astacusfluviatilis, obtained soon after casting its shell, and also its newly cast-off 

 covering, both of which are in excellent preservation. The following is a description 

 of the appearances observed in each : All the pieces of the exuvium are connected 

 together by the old articulations, and accurately represent the external form of the 

 complete animal j the carapace, or dorsal shield of the cephalo-thorax, alone being de- 

 tached, having been thrown off in one piece. The pedicles of the eyes and external 

 corneae, as well as the antennae, remain in situ, the corresponding parts having been 

 drawn out from them as the finger from a glove ; and no fissure of the shell or rupture 

 of the ligaments connecting the joints is anywhere visible in these portions of the skele- 

 ton. The auditory tubercles, and the membrane stretched over the orifice of the ear, 

 occupy the same position as in the living cray-fish. The jaws, foot-jaws, and ambu- 

 latory feet retain their original connections, with the exception of the right chela, which 

 had been thrown off before the moult began ; and the segments of the abdomen, false 

 feet, and tail-fin exactly resembled those of the perfect creature; even the internal 

 processes derived from the thoracic segments (apodemata) rather seemed to have had 

 the flesh most carefully picked out from among them, than to have been cast away from 

 a living animal : but perhaps the most curious circumstance observable was, that 

 attached to the base of each leg was the skin which had formerly covered the branchial 

 tufts, and which, when floated in water, spread out into accurate representations of those 

 exquisitely delicate organs. No fissure was perceptible in any of the articulations 

 of the small claws j but in the chela each segment was split in the neighbourhood 

 of the joints, and the articulating ligaments ruptured. The lining membrane of the 

 stomach was found in the thorax, having the stomachal teeth connected with it ; from 

 its position, it would seem that the animal had dropped it into the place where it lay 

 before the extrication of its limbs was quite accomplished. The internal tendons were 

 all attached to the moveable joint of each pair of forceps, both in the chela and in 

 the two anterior pairs of smaller ambulatory legs. 



On examining the animal, which had extricated itself from the exuvium described 

 above, the shell was found soft and flexible, but contained a sufficiency of calcareous 

 matter to give it some firmness, especially in the claws. The tendons of the forceps 

 were still perfectly membranous, presenting a very decided contrast when compared 

 with the old ones affixed to the discarded shell. The stump of the lost chela had not 

 as yet begun to sprout, and the extremity was covered by a soft black membrane. The 

 jaws were quite hard and calcified, as likewise were the teeth coutained in the sto- 

 mach. 



