A STUDY OF METAMEEISM. 897 



of three half- me tamer es united together? It is cue of the 

 aims of the paper to answer this question. 



To the other class of modifications belong the spirals, one of 

 the simplest of which is shown in fig. VII, a, b, c. The spiral 

 is produced by a combination of two half-compound metaraeres, 

 i.e. one raetamere is " split ^^ below, but the same one is 

 normal above. The next metamere is split above and on the 

 same side of the body, but not below. The posterior arm of 

 the anterior half-compound metamere is continuous at the 

 side with the anterior arm of the posterior (upper) half-com- 

 pound metamere. The same fact may be stated in a diff'erent 

 and perhaps a clearer way. A half-metamere on the right 

 side of the body unites in the mid-line below with a metamere 

 in front of it, and above with a metamere behind it, fig. 

 VII, c. We have formed as a result of this arrangement one 

 more half-metamere on the right side of the body than on the 

 left side, as is best shown by the construction in fig. VII, c. 



We may proceed to study more in detail the many kinds of 

 modification that group themselves around these two types. 

 We shall find that nearly all of the geometrical combinations 

 conceivable are to be found in the worms themselves. 



Category i^ fig. I, a, b, c. — This form has already been 

 described as the type. It is equally common on the right and 

 left sides of the body. 



Category ii, fig. II, a, b, c (above, below, and reconstructed, 

 as seen from above). — In this form one (above) of the lines of 

 the split turns to one side (posteriorly) to fuse with the septal 

 line. This leaves, as shown in the reconstruction, a free end 

 forming a modification of the compound metamere. This 

 modification may appear either on the right or left side of 

 the body, and the free end may occur above or below, anteriorly 

 or posteriorly. 



Category iii, fig. Ill, a, b, c. — By a simple shifting of the 

 lines in the last form we pass to the third category. Here 

 both of the lines of the extra half-segment turn towards the 

 same septal division, and we have formed an intercalated half- 

 metamere. 



