SEDGWICK 299 



and developed by later workers, particularly as regards 

 the relations to the coelom of the genital organs and 

 ducts and the nephridia, but no special methodological 

 interest attaches to these further developments. 1 We shall 

 here focus attention upon one interesting line of speculation 

 followed out in this country particularly by Sedgvvick the 

 theory of the Actinozoan ancestry of segmented animals. 

 Its relation to the Coelom theory lies in the fact that Sedgwick 

 regarded the segmentation of the body_as moulded upon the 

 segmentation of the mesoblast, which in its turn, as Kowa- 

 levsky and Hatschek had shown, was a consequence of its 

 mode of origin as a series of pouches of the archenteron. 

 In other respects Sedgwick's speculations link on more 

 closely to the Gastrasa theory, for one of his main con- 

 tentions is that the blastopore or Urmund is homologous 

 throughout at least the three metameric phyla. In follow- 

 ing up Balfour's observations on the development of 

 Peripatus? Sedgwick was struck with the close resemblance 

 existing between the elongated slit-like blastopore of this 

 form (giving rise to both mouth and anus), with its border 

 of nervous tissue, and the slit-like mouth of the Actinozoan 

 (functioning both as mouth and anus), round which, as the 

 Hertwigs had shown, there lies a special concentration of 

 nerve cells and nerve fibres. He found another point of 

 resemblance in the gastric pouches of the Actinozoa, which 

 he homologised directly with the enteroccelic pouches of the 

 Ccelomati. He was led to enunciate the following theses: 3 

 (i) that the mouth and anus of Vermes, Mollusca, Artho- 

 poda, and probably Vertebrata, is derived from the elongated 

 mouth of an ancestor resembling the Actinozoa ; (2] that 

 somites are derived from a series of archenteric pouches, like 

 those of Actinozoa and Medusae ; (3) that excretory organs 

 (nephridia, segmental organs) are derived from parts of these 

 pouches which in the ancestral form, as in many polyps, were 

 connected by a circular or longitudinal canal, and opened 



1 For an historical account of this work, see Lankester, loc, cit., pp. 

 21-37. 



2 Proc. Roy. Soc., 1883, and Q./.M.S., xxiii., 1883. 



3 " Origin of Metameric Segmentation," Q.J.M.S., xxiv., pp. 43-82 

 1884. 



