ORIGIN OF METAMERIO SEGMENTATION. 43 



On the Origin of Metameric Segmentation and 

 some other Morphological Questions. 



By 



Adam Sedgwick, Ifl.A., 



Fellow and Assistant Lecturer of Trinitj Collegej Cambridge. 



With Plates II aud III. 



In the following pages ^ certain hypotheses with regard to 

 the evolution of segmented Triploblastica (Annelida, 

 Arthropoda, Vertebrata), and some apparently unseg- 

 mented forms (Mollusca Brachiopoda, Sagitta, Balano- 

 glossus) are suggested and discussed. 



I have found it convenient to consider the Vertebrata speci- 

 ally in the latter part of the paper, because of the very pro- 

 nounced views which are held at the present day with regard to 

 their evolution. 



The paper is divided into two parts. The first part deals 

 with the evolution of certain organs ; the second part with the 

 evolution of the groups mentioned and especially with that of 

 the Vertebrata. 



My hypothesis concerning the origin of metameric segmen- 

 tation has been in a sense anticipated by Lang. He regards 

 the somites as derived from gut pouches such as are found at 

 the present day in Turbellarians. It should be remembered 

 that according to his view, the Turbellaria are specialised 

 Coeleuterates. My view of the origin of Somites differs from 



' A short account of the main points of this paper was comraunicaled to 

 the Cambridge Philosophical Society in November, 18S3, and published in 

 vol. V of the 'Proceedings' of that Society. 



