No. 415.] NEREIS VIRENS SARS. 573 
to the phenomenon of regulation observed by Rand (98,99), 
by which the more primitive ccelenterate, Hydra viridis, rids 
itself of supernumerary structures produced by abnormal re- 
generation. This inherent regulative power may have been 
assisted in the worm by arrested development of the inter- 
polated parts due to a deficient blood supply. The structure 
of the abnormal parapodia, as we have seen, warrants the 
assumption that their development is incomplete, and the 
defective condition of the vessels supplying these appendages 
strongly indicates that lack of sufficient nourishment was the 
cause. Such a defect in the vascular system, however, could 
merely aid in the reduction of the abnormal parts; their total 
disappearance must be due primarily to the profound regulative 
changes which took place in the reproducing anal metamere. 
Such a strong inherent tendency toward the production of a 
constant and normal condition undoubtedly accounts for the 
infrequent occurrence of abnormalities in regenerating worms. 
In most cases of injury, where regulation becomes necessary, 
the process is probably completed before the formation of new 
somites begins. When, on account of a specially serious injury, 
the process is delayed, partial duplications may be formed, but 
the regulative changes may still go on to completion and the 
normal condition be finally established, as in the present case. 
If, however, the traumatic influence is so profound as to give 
rise to separate regions of regeneration, regulation may take 
place in each and produce two normal series of metameres, 
but an abnormal bifurcated worm. 
From our description of this single case of abnormality in 
Nereis, and comparison of its structure with that found in bifid 
Polychzta, we may conclude : 
I. That a single metamere of a polychzete annelid may give 
rise to a complete or partial duplication of the main axis of the 
body 
?. The production of these abnormalities in all probability 
takes place in the post-embryonic worm, and is due to traumatic 
Stimulus, 
3. The Supernumerary, and in some cases useless, parts 
thus formed may be gradually suppressed by regulative changes 
