62 Mr Sedgwick, On the origin of segmented animals, etc. [Nov. 26, 



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

 (Vermes, Mollusca, Arthropoda, and in all probability Vertebrata) 

 have been derived from the mouth of an ancestor common to them 

 and the Ccelenterata, 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 resembling those found 

 at the present day in Actinozooid Polyps and Medusae. 



3. That the excretory organs or nephridia (segmental organs) 

 of the higher animals are derived from specialized 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 longitudinal canal (circular canal 

 of Medusod, perforations in mesenteries of Actinozoa) and with the 

 exterior by a pore 1 , 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 has persisted (segmental or pronephric duct) and retained its 

 posterior opening into the alimentary canal while the external 

 pores have been lost. 



The essence of these three propositions lies in the fact that the 

 segmented animals are traced back, not to a triploblastic unsegmented 

 ancestor but to a tivo-layered C eel enter ate-like animal with a pouched 

 gut, the pouching having arisen as a result of the necessity for an 

 increase in the extent of the vegetative surfaces in a rapidly enlarging 

 animal {for circulation and nutrition). 



The hypotheses are based upon the embryonic development of 

 the respective organs in the Triploblastica, and the structure of the 

 living Ccelenterata; in other words upon facts precisely of the same 

 nature as those which have been used in tracing the evolution 

 of the nervous and muscular tissues. 



Before proceeding to summarise the facts upon which the 

 hypotheses rest, I may be permitted again to point out that it is no 

 part of my view to derive segmented animals direct from the 

 Ccelenterata, but to derive both Ccelenterata and segmented animals 

 from a common Ccelenterate-like ancestor whose structure can only 

 be elucidated by studying the anatomy and the development of 

 the living Ccelenterates and of the higher segmented animals. 

 The main facts are shortly as follows. 



unable to quote them here, but he informs me that his paper is in the press and 

 will shortly appear in the Naples "Mittheilungen." 



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

 Jena. Zeitschrift, Bd. xni. 



