SECT. Yin THE CIRCULATORY SYSTEM 123 



the intersegmental folds, across which the dermal 

 blood-vessels of the original Annelid did not run (?). 

 These two modifications, (i) the degeneration of the 

 blood-vessels, and (2) the loosening of the peritoneum 

 from the body walls, are all that is needed to produce 

 the lacunar system of Apus from the blood vascular 

 system of an Annelid. 



The contrast between the circulatory systems of 

 the Annelida and of the Apodidae does not therefore 

 stand in the way of the acceptance of our theory of 

 their close relationship. Apart from the well-known 

 fact that, among the Invertebrata at least, blood 

 vascular systems have little morphological value, we 

 have here shown how simply the lacunar system of 

 Apus can be deduced from the blood vascular system 

 of an Annelid. 



We reserve any further discussion of the ccelom 

 epithelium, a part of which has been here used up in 

 the formation of the lacunar system, till we come to 

 the genital glands, when we shall again see what an 

 important part it has played in transforming the 

 Annelid into the Crustacean. (See further p. 169 and 

 Appendix III.) 



