312 
The animal was Nais lacustris, a Species in which the pro- 
stomium is greatly elongated forming a proboscis-like extension of the 
head. In this specimen there arises from the side 
of the prostomium a branch which is fully as long 
as the main prostomium, but a little more slender. 
The branch arises at a point some two thirds of the 
length of the main prostomium from the base and 
extends obliquely forward for some distance, then 
curving so that its tip lies directly in front of the 
tip of the main prostomium. Examination of the 
mounted specimen shows that the structure of the 
branch is in all respects similar to that of the 
main trunk. Unfortunately no observations were 
made upon the living animal concerning the position 
and use of the organ. 
It appears probable that this anomaly has arisen 
in consequence of some injury to the prostomium 
which has resulted in new growth at a point along 
the side. 
The kind of injury required to produce the result 
can be determined only by experimentation. It is possible that an 
injury upon one side of the prostomium might give rise to a growth 
of this kind. In consequence of such an injury regeneration pro- 
ceeding from the posterior side of the wound might lead to the 
formation of a branch like that figured. The inequality of the two 
branches would appear to indicate that they were not formed in 
consequence of the loss of the prostomium-tip. 
One point is of special interest: the branch is about as long as 
the main prostomium, i. e. a new prostomium of full lenght has arisen 
from a point near the tip of the original prostomium, just as it would 
arise if the latter were cut off at its base. If this be the result of 
regenerative processes, the lateral region of the prostomium, as well 
as its base, is capable under certain conditions of giving rise to a 
prostomium of full lenght. 
N 
EM 
Hull Zoological Laboratory, 
University of Chicago, February 1900. — 
