524 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
obtained as many divided specimens in the twelve hours following 
such changes of water as in all the other times of my experiments, 
although the ratio in time was only 1 to 10. This proportion can 
hardly be considered an accident, but I think the point is in need of 
more careful investigation. 
All that these experiments indicate is, that the worms will not 
divide unless well fed, and that even when they are ready for divi¬ 
sion it may be prevented by an abnormal condition such as the 
foulness of the water. This emphasizes the necessity of having all 
the conditions as favorable as possible in any such experiments. 
In the laboratory the fission occurred betAveen 10:00 p. m. and 
6:00 A. M. in thirty-nine specimens out of forty-two in which the 
approximate time of division was recorded. This did not seem due 
to the amount of light to wdiich the animals Avere subjected during 
the day, for some of the dishes Avere so shaded that there was 
practically no light, day or night, except when they were being 
examined, and the division was the same in these as in others which 
Avere exposed to the full daylight. Lehnert (’91) observed in 
Bipaliiim hewense that the division took place at night. 
In trying to show that the fission occurring in the laboratory is a 
normal reproductive process I have taken pains to show that it 
occurs to an e\"en greater extent in nature. To do this I collected 
some three thousand worms from different localities and found them 
with the marks of more or less recent division always abundant. 
The following count of the specimens collected at one time is repre¬ 
sentative. Whole, 75; heads, 110; tails, 60; odd mutilations, 
evidently the result of collecting, 5. These figures are good in 
asexual localities at any time from June to October, and when it is 
known that tail pieces may redivide after five days and heads after 
about tAvice that time, one realizes hoAV much multiplication may be 
thus effected in a short time. A smaller number of tails is con¬ 
stant in about tlie above proportion. The tails are also in the 
minorit}^ in the laboratory unless isolated, from the fact that they 
are eaten by the whole Avorms and their own head ends, and even 
Avhen isolated they sometimes die apparently from becoming 
entangled in their own mucous secretions. As they are without 
the pharynx or sense organs of the head (pi. 9, figs. 3, 12, 13 ; pi. 
10, fig. 21; pi, 11, fig. 25), there is probabl}^ more destruction of 
tails than of heads in nature where they may be preyed upon by 
