SPIRAL MODIFICATION OF METAMERISM. 



T. H. MORGAN. 



The unusual forms of metamerism recorded below are by no 

 means uncommon in the Annelids. They are to be found in 

 the most widely separated groups, but for briefness and sim- 

 plicity of treatment I confine myself here to the more important 

 results obtained on Allolobophora fcetida. 



In a lot of 318 worms, 218 were found normal with respect to 

 the metameres, and 100 abnormal. That is to say, in the pro- 

 portion of about one to two, or, one worm out of each three was 

 abnormal. 



These abnormalities were of several kinds, some of these 

 being described below. Many of the worms showed more than 

 a single abnormality. Thus out of the above hundred : — 



With I abnormality 65 



" 2 abnormalities 16 



3 " 10 



" more than 3 " o 



One of the commonest forms of abnormality, and also the sim- 

 plest, is what we may speak of as the split-metamere, although 



C 



^ 



a be 



Fig. I. 



the term is, as I hope to show, misleading. This is shown in 

 Fig. I, a, by c, giving the dorsal and ventral views of several 

 rings. Fig. i a shows the dorsal view, b the ventral, and c an 

 imaginary construction representing the split-metamere as 

 opened ; the dotted lines represent the under surface. The 

 middle metamere of the three is split. 



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