86 THE CHANGES PROM STAGE A TO E. 



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

 return later), from its development (Ziegler, No. 53) an 

 enteroooelej 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. Tlie chief difference, I take it, between the pseudo- 

 coele (body cavity, pericardium, and heart) 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 

 iu 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 pai'ts 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 papillae, or third pair. 



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



5. The somites of the anal papillae, or twenty-first pair. 



6. The rudimentai'y somites behind the twenty-first. 



The heading 4 will have to be further subdivided according 

 as the dorsal divisions contain generative cells or not. 



The Somites of the First Pair (somites of pre-oral or an- 



