DEVELOPMENT OF THE CAPE SPECIES OP PERIPATUS. 503 



identically similar to that of the ventral part of the median 

 chamber of the body cavity. The whole system is probably in 

 communication and functions as a vascular space of which the 

 heart is a specially marked off and contractile tract. The 

 body cavity and pericardium of Peripatus^ if comparable 

 with anything in Annelida or Mollusca, must be looked 

 upon as homologous with the vascular system. The peri- 

 cardium of a Mollusc is, I think (but to these points I shall 

 return later), from its development (Ziegler, No. 36) an 

 enteroccele, and as such has no communication — in this respect 

 resembling the remains of the enterocoele of Peripatus — with 

 the pseudocoele represented by the heart and vascular system 

 and spaces. The chief difference, I take it, between the pseudo- 

 coele (body cavity) of Peripatus and the pseudocoele (vascular 

 system and vascular spaces) of a Mollusc is that the latter is 

 usually largely broken up by anastomosing strands of muscle 

 and connective tissue, while in Peripatus the same space is, 

 except in the legs, a fairly continuous and unbroken system. 



If the above suggestions are correct, and if at the same time 

 the body cavity and heart of other Arthropoda develope in 

 the same manner as in Peripatus (I shall examine this ques- 

 tion later), then that peculiar Arthropod feature, viz. the paired 

 ostia leading from the heart into the pericardium, receives a 

 morphological explanation. 



Further Account of the Development of the Somites. 



After the foregoing description the various parts of the figures 

 will be intelligible, and I may proceed to give an account of 

 the changes which take place in the other somites. In doing 

 this I shall refer to the changes in the mesodermic tissue 

 generally. The somites, with regard to their development as 

 far as Stage f, may be grouped under six heads : — 



1. The somites of the pre-oral lobes, or first pair. 



2. The somites of the jaws, or second pair. 



3. The somites of oral papill£e, or third pair. 



4. The somites of legs 1 — 17, or fourth to twentieth pair. 



VOL. XXVII, PART 4. NEW SEK. N N 



