ARTHROPODA 



315 



only by the cavities of the gonads and of certain excretory organs and 

 occasional vestiges elsewhere. 



The excretory organs of arthropods are of very various kinds. True 

 nephridia appear never to be present. Coelomoducts are present in a 

 number of cases, though in the absence of perivisceral coelom they 

 end internally each in a small coelomic vesicle or "end sac". These 

 are found in the Onychophora in a long series of segmental pairs. In 

 Crustacea there is either a pair of coelomoducts on the third (an- 

 tennal) somite or a pair on the somite of the maxillae, or, rarely, both 



mdm 



ect. 



Fig. 213. Early stages in the development of Astacus. After Morin and 

 Reichenbach. A-C Cleavage, D gastrulation. a. anterior end of embryo; 

 blp. blastopore; ect. ectoderm; end. endoderm; mdm. mesoderm; nu, nuclei; 

 pt. posterior end of embryo; v, yolk; yp, "yolk pyramids", due to the 

 transitory appearance of divisions of the yolk corresponding to the superficial 

 cells. 



these pairs are present. In various crustaceans other glands, some 

 ectodermal, some mesodermal, appear to have an excretory function, 

 and sometimes replace both pairs of coelomoducts, which become 

 vestigial. In arachnids, coelomoducts open on one or two of the pairs 

 of legs. They are known as coxal glands, but are not homologous with 

 the glands to which that name is applied in certain crustaceans. Mal- 

 pighian tubules are tubular glands which open into the alimentary 

 canal near the junction of mid and hind gut in the Arachnida, Insecta, 

 and Myriapoda. In archnids they, are of endodermal origin, but in 

 insects and myriapods they are part of the ectodermal hind gut. It 

 is interesting that the subphyla differ in the nature of their nitro- 

 genous excreta. In the Crustacea these are principally ammonia 

 compounds and amines, in the Insecta they are urates, in the Arach- 

 nida guanin. 



