ORIGIN OF METAMERIC SEGMENTATION. 93 



very forcibly the hypothesis that the mesoblastic somites of 

 segmented animals are derived from a diploblastic Ccelenterate- 

 like ancestor with folded gut walls, the folding having arisen 

 as a result of the necessity for an increase in the extent of the 

 vegetative surfaces in a rapidly enlarging animal. 



I would venture, therefore, to suggest that Medusae, 

 Actinozoa and segmented animals are all derived from a 

 common diploblastic ancestor, the Gastrsea; that as this 

 Grastrsea increased in size it became necessary that some 

 arrangement should arise by which a proper circulation of the 

 nutritive matter to all parts of the body should be effected. 

 For this purpose the gut wall became folded in such a way as 

 to give rise to the radial and circular canals of Medusae; to 

 the mesenterial chambers (communicating peripherally by 

 mesenterial stomata) of Actinozoa, and to the pouched diplo- 

 blastic form from which segmented animals have arisen (I do 

 not mean to assert that the segmented animals are the only 

 animals which have arisen from a diploblastic animal with a 

 pouched gut ; vide below p. 94). 



In a segmented animal the mesoblast is the first part of the 

 body to show segmentation. The rest of the segmentation is 

 moulded on the segmentation of the mesoblast. That is to 

 say, the segmented organs, primitively at any rate, correspond 

 in their segmentation with the somites. For each somite 

 there is the nephridium, nerve ganglion, &c. 



Supposing there is anything in the hypothesis I am putting 

 forward, viz. that the somites of segmented animals are derived 

 from gut pouches, which are homologous with the alimentarjr 

 pouches of Coelenterata, then it ought to be possible to explain 

 on the same hypothesis the similar repetition of other organs. 



In a segmented animal the following organs usually show 

 the same repetition as the mesoblastic somites ; the external 

 appendages, the nephrida, the muscular system and the nervous 

 system. 



In Coelenterata, both in Medusae and Actinozoa — 



(1) The tentacles correspond as a rule to the radial canals or 

 to the mesenterial pouches ; 



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