xi PHYLUM ARTHROPODA 547 



large species (6-8 cm. in length) parasitic in the mouths of Fishes, 

 where they hold on to the mucous membrane with their short clawed 

 legs : their mouth-parts are often modified for sucking. In the Bopy- 

 rini, found in the gill-cavities of various Crustacea, parasitism is 

 accompanied by great degeneration and asymmetry (3), as well as 

 by a notable degree of sexual dimorphism, the males (3, b,m) being 

 very small and permanently attached to the bodies of the females. 

 Lastly, in Cryptoniscus, parasitic on Crabs, the adult female (^, b) has 

 no trace of crustacean organisation, and it is only by the study of 

 development that its true systematic position can be guessed. 



With regard to the texture of the exo skeleton, there is every 

 gradation from the delicate polished cuticle of most Entomostraca, 

 Schizopods, &c., through the calcified but still flexible cuticle of 

 Astacus, to the thick, tuberculated, stony armour of many Crabs 

 (Fig. 428, 3) or the shelly pieces of Cirripedes. The exoskeleton is 

 secreted from a single-layered ectoderm, and undergoes periodical 

 moults or ecdyses. There is no transverse 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 r 

 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 gonadsare also 

 coelomic, and the ducts by which they communicate with the 

 exterior are probably modified nephridia. 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 pericardial 

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

 constitute a kind of secondary body-cavity, formed by the enlarge- 

 ment of blood-vessels, which have largely replaced the true ccelome. 

 Such a secondary or blood-containing body- cavity is called a 

 hcemoccele. 



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 Entomostraca, part or the whole of the stomach is formed 

 from the mesenteron, but in Malacostraca both gullet and stomach 

 are developed from the stomodaBum. 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 Arthrostraca (Fig. 435, I) they are 

 unbranched caeca 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 



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