246 



MORGAN. 



[Vol. VII. 



On one side (left in a) of the split-metamere the usual condi- 

 tions prevail, while on the other (right in a) all of the important 

 structures are doubled, — setae, nephridia, etc. There is a half- 

 septum with double walls corresponding to the split. Twenty- 

 eight cases of simple splitting were found in this lot. 



The most interesting abnormality and also the most frequent 

 is what I have spoken of as spiral metamerism. Thirty-seven 

 such cases were counted in the one hundred abnormalities given 

 above. These spirals are of all lengths, from the simple spiral 

 involving a few metameres to those involving more than twenty 

 metameres. The highest degree of complication amongst these 

 spiral forms comes in what I shall speak of as the double and 

 triple spirals. 



The accompanying diagram (Fig. 2, a, b, and c) illustrates a 

 spiral of a medium length. The spiral begins with a split-meta- 



FiG. 2. 



mere. The upper (right) division of the split forms a ring below 

 with the whole of the other (left) side, in this way leaving free 

 the lower division of the split, which begins to wind around the 

 body as shown in c, passing completely around four times, and 

 ending in the split-metamere at the bottom. Here the spiral 

 comes to an end in a split-metamere opening in the reverse 

 direction from the upper split. All combinations between the 

 split-metameres and the spirals that are geometrically possible 

 (and there are several variations) are actually found in the 

 worms, provided a sufficient number of forms be examined. 



