CCELOM OP AETHROPODA. 143 



condition of the coelom and vascular tracts in the adult and 

 the relation of the coelom to the vascular body cavity is clearly 

 illustrated by the diagram (PL XIII, fig. 17). It is commonly 

 said that in the Arthropoda the generative ducts are continu- 

 ous with the glands, and in this they are contrasted with the 

 Annelida and Vertebrata. As a matter of fact, however, the 

 generative ducts, in Peripatus at least, present exactly the 

 same relation to the generative glands as do the oviducts of a 

 dogfish or earthworm to the ovaries of those animals ; that 

 is to say, like the latter, the generative ducts open into the 

 coelom, and the ova are products of the coelomic epithelium. 



It is important to notice that in Peripatus the nephridia 

 are parts of the coelom (PI. XIII, diagram, fig. 17), just as 

 they are in Elasmobranchs. They are commonly spoken of in 

 a manner which implies that they have but little to do with 

 the coelom beyond opening into it. This way of speaking 

 of them is calculated to mislead. The nephridia are 

 direct differentiations of part of the coelom (vide 

 diagrams, figs. 13 — 17, and figures illustrating their develop- 

 ment). 



A negative feature, which has often been put forward as 

 characteristic of the Arthropoda, is the apparent absence of 

 nephridia. The nephridia of Peripatus have generally been 

 considered as a primitive and peculiar feature. Lankester,^ 

 however, some time ago (No. 54, p. 516), suggested that the 

 coxal glands of Limulus and the antennary glands of Crus- 

 tacea were nephridia, and that the peculiar " end-sacs " 

 described by Gulland in the coxal glands of the young 

 Limulus, and the internal vesicle of the Crustacean anten- 

 nary gland described by Grobben (No. 55), were part of a true 

 coelomic space. The discovery of the end-sacs in Peripatus, 

 and of their method of development, seems to confirm Lan- 

 kester's view. And it is interesting to notice that the end- 

 sac of the Crustacean green gland, as figured by Grobben, 

 resembles somewhat in the structure of its wall the end-sacs of 

 the Peripatus nephridia. 



* See above, p. 110. 



