A STUDY OP METAMERISM. 429 



wedged in ventrally between two segments, while the other 

 left half completes the ring. We have here evidently a modi- 

 fication of the compound metamere similar to category ii, 

 PI. 40, fig. II. 



In PI. 43, fig. 83, a, b, we see a metamere imperfectly deve- 

 loped on the right side. The dorsal parapodium and gill are 

 absent on this side, but the ventral parapodium is present, as 

 seen in side view in b. We have here probably a case of 

 iuc>)mplete development of a metamere of one side. 



Another somewhat similar modification is shown in fig. 84, 

 A B. The first of these shows the ventral side, and the middle 

 segment of the three drawn is seen to be imperfectly deve- 

 loped. The ventral parapodium of the right side (left of 

 figure) is present, but the metamere does not reach as far 

 dorsally as the line of dorsal parapodia (see b). On the left 

 side of the body we find the metamere completely undeveloped. 

 We seem to have here a half-segment of the right side that 

 has not fully developed even on that side, but has overgrown 

 the mid-ventral line on the left side. 



Fig. 85 shows a spiral beginning and ending on the dorsal 

 surface. Seven segments are involved, giving five turns to 

 the spiral. 



The large number of abnormal forms found in Amphinome 

 is in part due no doubt to the frequent regeneration of por- 

 tions of the body that takes place. The segments are broad, 

 and for this reason it is surprising to find abnormal com- 

 bination of the metameres so frequent. In other poly- 

 chsetous Annelids, where the metameres are very narrow, the 

 false unions and imperfections of the metameres are very 

 numerous. 



Cori has figured several modifications, and I have found 

 similar ones that are exceedingly difficult to explain as simply 

 due to modifications of half-metameres. Such cases are 

 particularly common in polychsetous Annelids. 



The majority of these very abnormal forms are, I think, due 

 to regeneration rather than to egg-variation. Since the method 

 of regeneration is more irregular than the growth of the 



