i6 



THE CRUSTACEA 



Excretory Sj/stem. — The most important organs of renal excre- 

 tion in the Crustacea are two pairs of glands, lying at the base of 

 the antennae and of the maxillae respectively, which are probably 

 the survivors of a series of segmentally arranged coelomoducts 

 present in the primitive Arthropoda. The two pairs are never 

 fully developed at the same time in one individual, although one 

 may replace the other in the course of development. Thus, in the 

 Branchiopoda, the antennal gland develops early and is functional 

 during a great part of the larval life, l>ut it ultimately atrophies 

 and the maxillary gland takes its place as the excretory organ of 

 the adult. In the Decapoda, where the antennal gland alone is 

 well developed in the adult, the maxillary gland sometimes pre- 

 cedes it in the larva. The structure of both glands is essentially 

 the same (Fig. 10). There is a more or less convoluted glandular 



tube (/), of mesoblastic 

 con origin, connected internally 



witli a closed " end-sac " 

 (e.s), representing a vestigial 

 portion of the coelom, and 

 generally a thin-walled duct 

 which opens to the exterior. 

 In the Branchiopoda the 

 maxillary gland is lodged 

 in the thickness of the 

 shell -fold (when this is 

 present), and from this 

 circumstance it often 

 receives the somewhat mis- 

 leading name of " shell- 

 gland." In the Decapoda, 

 the antennal gland is largely 

 developed and often very 

 complex, and is known as 

 the "green gland." 

 Other excretory organs have been described in various 

 Crustacea, but although their excretory functions have been 

 demonstrated hy physiological methods, their morphological rela- 

 tions are in most cases quite obscure. In some cases they consist 

 of masses of mesodermal cells, within which the excretory products 

 are stored up instead of being expelled from the body. In other 

 cases an excretory function is attributed to certain cells of the 

 mesenteron or to some of its diverticula. 



Nervous Si/slein. — The central nervous system is constructed on 

 the same general plan as in the other Arthropoda, consisting of a 

 supra-oesophageal ganglionic mass or " Iji-ain," united by circum- 

 oesophageal connectives with a double ventral chain of segmentally 



Fig. 10. 



Antennal gland of a larva of Estlirrin (Branchio 

 poda). (After Grobben.) cow, connecti\-e-tis.sue fibre 

 e.s, end-sac ; o, external oijening ; t 



,dandular tubule. 



