80 ADAM SEDGWIOK. 



pore and somites, and an adult Actinozooid polyp. I 

 pointed out that the comparison of these two structures sug- 

 gested an explanation^ which so far as I know has not before 

 been suggested, of a great morphological difficulty, viz. the 

 origin of metameric segmentation (vide ' Nature,' December 

 28th, 1882). At the same time I pointed out that by follow- 

 ing up this comparison some other morphological difficulties 

 received an explanation. 



The hypotheses I suggested were shortly as follows : 



1 . The mouth and anus found in most of the higher groups 

 (Vermes, MoUusca, Arthropoda and in all probability 

 Vertebrata) have been derived from the mouth of an ancestor 

 common to them and the Coelenterata ; i. e. from an elongated 

 opening such as is found at the present day in the Actinozoa. 



2. That the somites of segmented animals are derived from 

 a series of pouches of the primitive gut (archenteron) of a 

 Coelenterate-like ancestor, i.e. from pouches generally resem- 

 bling those found at the present day in Actinozooid polyps 

 and Medusae. 



That the excretory organs or nephridia (segmental organs) 

 of the higher animals are derived from specialised parts of 

 these pouches which were in the supposed ancestor, as indeed 

 they now are in many living Medusae and Actinozoid 

 Polyps connected peripherally with each other by a longi- 

 tudinal canal (circular canal of Medusae, perforations in mesen- 

 teries of Actinozoa,) and with the exterior by a pore^ one 

 for each pouch; further, that in the Invertebrata, e.g. 

 Annelida, the longitudinal canal has been lost and the external 

 pores retained, while in the Vertebrata the longitudinal 

 canal (segmental or pronephric duct) has persisted and 

 retained its posterior opening into the alimentary canal while 

 the external pores have been lost. 



I now add to these three propositions a fourth. 



4. That the tracheae are not developed from cutaneous glands 



' Vide Hertwig, 'Organismus der Medusen,' p. 39; and "Actinien," 

 ' Jena Zeitschrift,' Bd. xiii. 



