ORIGIN OF METAMERIC SEGMENTATION. 57 



The Triploblastica and the Actinozoa are descended from a 

 common two-layered bilateral ancestor which possessed an 

 enlarged oral surface^ an elongated mouth opening which by 

 the adhesion of its middle portion was functionally divided 

 into two openings, one at each end of the loug axis of the 

 mouth. The nervous system was generally distributed on the 

 ectoderm all over the body, but was probably, as in living 

 Actinozoa, especially concentrated on the oral surface. This 

 type has persisted with certain modifications in Actinozoa, but 

 in Peripatus and the other triploblastic forms under discussion 

 the primitive mouth has completely divided, the body has elon- 

 gated, and the nervous system has become especially aggregated 

 in a ring (as in Medusae) round the mouth and anus. 



On the Origin of Metameric Segmentation. 



It has for some time been recognised that the body cavity or 

 ccelom of the Triploblastica has been derived from diverticula 

 of the archenterou. Such diverticula have been known for 

 some time in the Echinodermata, Sagitta, Brachiopoda, 

 Balanoglossus, Amphioxus. 



The development of the body cavity in Annelida, Arthro- 

 pod a, Vertebrata, and other coelomate forms without diver- 

 ticula has been supposed to be an embryonic abbreviation of 

 this primitive process. I may quote the following passages 

 from Balfour on this head. 



''The formation of hollow outgrowths of the archenteron, 

 the cavities of which give rise to the body cavity, can onlv be 

 explained on the supposition that the body cavity of the types 

 in which such outgrowths occur is derived from diverticula 

 cut off from the alimentary tract. The lining epithelium of 

 the diverticula, the peritoneal epithelium, is clearly part of the 

 primitive hypoblast, and this part of the mesoblast is clearly 

 hypoblastic in origin. . . . There can be but little doubt 

 that the mode of origin of the mesoblast in many Vertebrata 

 as two solid plates split off from the hypoblast, in which a 

 cavity is secondarily developed, is an abbreviation of the 

 process observable in Amphioxus ; but this process approaches 



