CCELOM OF ARTHEOPODA. 119 



across the middle dorsal and ventral lines, the separating 

 walls between successive somites for the most part persisting ; 

 they constitute the ccelomic body cavity of Lumbricus. 2. In 

 Peripatus the vascular channels, excepting the heart, are swollen 

 out to wide channels, more or less completely continuous with 

 one another, so as to form four or five main vascular tracts, 

 while in Lumbricus they are present minute, branching, well- 

 defined as canals. 



On comparing Peripatus with other Arthropodain this con- 

 nection we are at once met with these facts : (1) that in no other 

 Arthropod, with the possible exception^ of certain Crustacea 

 and of Limulus (Lankester, No. 54), are nephridia, recognisable 

 as such, present ; (2) that the cavities of the somites cannot be 

 traced beyond a comparatively early stage of development; 

 (3) that the early stages of the generative organs have not 

 been thoroughly made out. 



We may, however, with fair probability predict, from what 

 we know (1) of the development of Peripatus, and (2) of the 

 resemblance of its body cavity to that of other Arthropods, 

 that when the development of the latter has been fully worked 

 out it will be found — as has been suggested by Lankester 

 (No. 54, p. 517) — that the coelom of the embryo persists as 

 the generative tubes and their ducts, but for the most part 

 vanishes (possibly giving rise to glands of a doubtful nephridial 

 nature), and that the body cavity and vascular system has an 

 exclusively pseudocoelic origin. 



> The coxal glands of the young Limulus, as described by GuUand (No. 

 54), and the antennary glands of Crustacea, as described by Grobbea (No. 

 55), end internally in a structure, which closely resembles the internal vesicles 

 of the nephridia of Peripatus. Lankester, in 1885, commented on the 

 " end-sacs " of the coxal glands of the young Limulus as follows (No. 54, 

 p. 516) : — " The observations here recorded on the structure and connec- 

 tions of the immature coxal gland of Limulus tend to render it probable 

 that the green glands of Crustacea are also to be regarded as a pair of 

 modified nephridia ;" and he goes on to say that, " it seems not improbable 

 that the so-called ' end-sac ' of these glands is not part of the nephridium. 

 but is developed from the connective tissue space (ccelomic space) into which 

 he true tubular nephridium originally opened." With slight modifications 

 this view is entirely borne out by the discovery of the "end-sac" and its 

 mode of development in Peripatus. 



