64 Mr Sedgwick, On origin of segmented animals. [Nov. 26, 1883. 



Marginal pores in Medusae. Development of excretory organs 



Openings in body wall of Ac- in Elasmobranchs and other Yerte- 

 tinia. brates 1 . 



Development of excretory organs 

 in Polygordius (Hatschek). 



To sum up in a few words : the Coelenterata differ from seg- 

 mented animals only in the fact that the alimentary or archenteric 

 pouches (mesoblastic somites) and the alimentary canal do not 

 become separate, and connected with this absence of a distinct 

 coelom is the low state of differentiation of such ccelomic structures 

 as the excretory organs and the absence of a separate vascular 

 system. 



In conclusion I may say that I do not put forward these 

 hypotheses in a dogmatic spirit, and that I fully recognize that 

 theories dealing with the complicated facts of Morphology can only 

 have in most cases a very temporary value. The hypotheses just 

 discussed first occurred to me some years ago when investigating 

 the development of the vertebrate excretory organs, and they have 

 received such striking confirmation from Hatschek's work on 

 Amphioxus and more recently from the embryo of Peripatus 

 Capensis that I have at length decided to publish them, hoping 

 that they may at least excite criticism, and so lead to the increase 

 of our knowledge and to the greater definition of our ideas. 



A full discussion of the facts bearing upon the views advocated 

 in this paper, illustrated by explanatory diagrams, together with 

 some speculations (following the same lines) on the origin of 

 tracheae and gill slits, and on the evolution of the vertebrata, will 

 I trust appear in the January number of the Quarterly Journal 

 of Mic. Science. 



1 Vide Sedgwick, "Development of Kidney, etc." Quart. J. Mic. Sci. Vol. xx. 

 1880, and "Early development of Wolffian duet, etc." Hid. Vol. xxi. 1881. 



