xi PHYLUM ARTHROPODA 593 



maining three thoracic appendages (6 8) are slender legs pro- 

 vided with exopodites : the last of them has a styliform copu- 

 la tory organ (p) developed from its proximal segment. The 

 pleopods are large and biramous : the first five (pi, po) have gill- 

 filaments (b)-) attached to their plate-like exopodites : the sixth 

 (pO} form large uropods or lateral tail-lobes, as in Astacus. 



With regard to the texture of the exoskeleton, there is 

 every graduation from the delicate polished cuticle of most 

 Branchiopoda, Ostracoda, Copepoda. &c., through the calcified but 

 still flexible cuticle of Astacus, to the thick, tuberculated, stony 

 armour of many Crabs (Fig. 469, J), or the shelly pieces of Cirri- 

 pedes. The exoskeleton is secreted from a single-layered ectoderm, 

 and undergoes periodical moults or ecdyses. There is no trans- 

 verse layer of muscle, and the longitudinal layer is broken up 

 into paired dorsal and ventral bands. As a rule, each limb- 

 segment is acted upon by two muscles : the joints are nearly 

 always hinge-joints. 



The body-cavity consists of several chambers separated from 

 one another by partitions. In Palcemonetes, one of the Prawns, 

 there is a median dorsal chamber enclosing the ophthalmic artery, 

 and not containing blood : it is probably a portion of the coelome 

 in the strict sense of the word. The cavities of the gonads are 

 also coelomic, and the ducts by which they communicate with the 

 exterior are probably modified coelomoducts. In addition to these 

 cavities there is a large central space, in which the enteric canal, 

 digestive glands, gonads, &c., lie ; paired lateral spaces containing 

 portions of the shell -gland ; spaces in the limbs ; and the peri- 

 cardial sinus, in which the heart lies. All these cavities contain 

 blood, and constitute a kind of secondary body-cavity, formed by 

 the enlargement of blood-vessels, which have largely replaced the 

 true coelome. Such a secondary or blood-containing body-cavity 

 is called a hccmoccclc. 



The enteric canal consists of a vertical gullet, an expanded 

 " stomach," and a nearly straight horizontal intestine. In some of 

 the Cladocera the intestine is coiled, but this is quite exceptional. 

 In the lower Crustacea, part or the whole of the " stomach ' is 

 formed from the mesenteron, but in Malacostraca both gullet and 

 " stomach ' (gizzard) are developed from the stornodseum. A 

 " gastric mill ' is present in Malacostraca, and a rudiment of 

 such an apparatus occurs in Ostracoda. The digestive glands 

 are usually branched caeca formed as offshoots of the mesenteron: 

 in the Isopoda and Amphipoda (Fig. 471, /) they are unbranched 

 ca?ca extending into the abdomen : in Stomatopoda they consist 

 of ten metamerically arranged organs opening into the intestine. 

 In Amphipods there are intestinal caeca (ud) which may have an 

 excretory function. So-called salivary glands, opening on the 

 labrum, have been found in several genera. 



