DEVELOPMENT OF THE CAPE SPECIES OP PEPtlPATUS. 535 



of posterior somites. The pericardium, heart, whole of the 

 body cavity (central, lateral, and leg compartments) are exclu- 

 sively pseudoccelic in origin. 



In Peripatus, therefore, the gonads are coelomic, and their 

 ducts what Lankester would call nephrodinic. 



The condition of the body cavity and ccelom of Peripatus 

 will be best appreciated by comparing it with that of the same 

 organs in an Annelid, such as Lumbricus. 1. InLumbricus 

 the structures corresponding to the nephridial vesicles of 

 Peripatus have swollen up and united with one another in pairs 

 across the middle dorsal and ventral lines, and after some 

 time have become united with one another longitudinally, 

 though the separating walls between successive somites for the 

 most part persist ; they constitute the coelomic 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 

 as minute, branching, well-defined canals. 



On comparing Peripatus with other Arthropod a in this con- 

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

 Arthropod are nephridia, recognisable as such, present ; (2) that 

 the cavities of the somites cannot be traced beyond a compara- 

 tively 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 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 pseudoccelic origin. 



In the Mollusca the coelom and vascular space have no4; 

 been generally sufficiently distinguished from one another. 

 There seems, however, to be no doubt that the pericardial 



VOL. XXVII, PART 3. NEW SER. V P 



