J5Y K. J GODDARD. 53 



•the annuli, and the significance of the same as bearing on the 

 generic importance of that annulation. In my studies in this 

 connection I have assumed the uniannulate condition as being 

 the primitive one, and in this subject for consideration the 

 question is raised as to which annulus of the somite represents 

 potentially that hypothetical primitive ring. 



Tn many leeches there occur, on special annuli, certain sensory 

 ~ papilliB which are more important and prominent than au}^ others 

 which may be developed on the remaining annuli, and these 

 papillae serve as an indication of the somite-arrangement and 

 constitution. Among other externals which are of the same 

 assistance may be mentioned the nephridiopores, whose position 

 relative to that of the main sensory papillae is constant. It was 

 ■only natural that these two structures should have been taken by 

 •earlier workers as external signs of the metamerism, as it was 

 read ily corroborated by a study of the central nervous system; and 

 it can be readily understood that Whitman should have assumed 

 that the annulus bearing the sensory papillae represented the first 

 ring of a somite (this annulus carrying also the nerve ganglia), 

 and that the nephridial aperture lay in the last annulus of the 

 .somite. Whitman's idea in this connection was upheld until 

 Castle, in 1900, came to the conclusion, from a detailed study of . 

 •the nervous system, that the limits of a somite were to be recog- 

 nised from a knowledge of the neuromerism, and from a study of 

 this he came to the conclusion that the sensory annulus repre- 

 sented in general not the first but the middle or potentially 

 middle annulus of the somite. He has further worked out the 

 order of abbreviation, etc., at either end of the body, pointing 

 out that the sensory ring was the most stable of the component 

 annuli of the somite, and that the other annuli were in the first 

 place derived from this sensory annulus by divisions of it 

 anteriorly and posteriori}'. 



Castle's conclusions have, in the main, been supported very 

 strongly by l^ivanow's excellent detailed work on the nervous 

 system. 



