396 T. H. MORGAN. 



these abnormal conditions, one by C. J. Cori (13), and the 

 other by the present writer (33). Both writers pointed out 

 the general interest attached to these modifications, and their 

 importance for an interpretation of the general problem of 

 metamerism. Subsequently a third paper appeared (10), re- 

 cording the presence of similar abnormalities in many groups 

 of Annelids, but without making any attempt to solve the 

 problem itself. 



My own paper was only a preliminary notice, and until the 

 present time I have not had an opportunity of fully describing 

 the material that I had at that time already accumulated, 

 studied, and drawn. The present paper attempts to give a 

 full consideration of the facts only touched on before, and to 

 extend over a wider field the conclusions reached. 



The modification of the rings, segments, or metameres of 

 the Annelid fall into two general classes, — not, however, 

 sharply separated from one another. 



The first of these I shall speak of as the compound metamere, 

 in contradistinction to the normal or simple metamere. In 

 the earlier paper the shorter term '^ split metamere" was 

 used, although it was there pointed out that the term was 

 misleading. 



The other type of modification may be spoken of as the 

 spiral metamere or spiral modification, or briefly as the spiral. 



The simplest case of the compound metamere is represented 

 by the diagram on PI. 40, fig. I, a, b, c. Fig. I, a, is a dorsal 

 view of a compound segment. The segment is double on the 

 right side, normal on the left. Fig. I, b, is the same, turned 

 over so as to be seen from below, showing a similar doubling 

 below on the same (right) side of the body. Fig. I, c, repre- 

 sents the compound segment as opened along the " split." It 

 is conceived to be transparent, and the dotted line to represent 

 the lower surface outlines. The general appearance is that a 

 metamere has been split into two on one side of the body, but 

 has retained on the other side its normal structure. The 

 question at once arises, are we dealing here with a case of 

 division of a metamere on one side of the body, or is it a case 



