CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 523 
(effect of irritation here and in the land planarians led me to try the 
effect of irritating Planaria maculata in various ways. 
The killing fluids used never caused whole worms to separate, as 
frequently happens with the land planarians. No amount of irrita¬ 
tion with blunt instruments produced any result. Many small cuts 
at different places on the surface, transverse cuts which almost 
divided the specimen at the place where normal division occurs, or 
at other points, and irregular mutilations at different places, are all 
quickly healed and do not produce any more divided specimens in 
a lot of twenty or twenty-flve worms than are to be found from day 
to day in lots which have not been thus mutilated, and it is the 
same when single specimens are isolated. According to my records, 
the division was even less in lots so treated. It seems, therefore, 
that ordinary irritation will not produce the division, which must be 
considered a spontaneous andp)erfectly normal process serving the 
purpose of midtiplication. The facts which were noted at the begin¬ 
ning of this paper, upon the absence of reproductive organs in worms 
from localities where flssion occurs extensively, further demonstrate 
that the multiplication of the species in such localities must be accom¬ 
plished almost entire-ly by this asexual method. 
The only influences by which I succeeded in regulating the fission 
were food and a fresh supply of water. These were effective only 
in a general way. If twenty or twenty-five worms are isolated and 
not fed, some of them will be found divided from time to time dur¬ 
ing the first three or four days. They may be kept in this w'ay for 
many weeks, but after the first few days, even though the water is 
frequently changed, the divisions are so rare as to be practically nil 
and might easily be explained by their obtaining food from eating 
one another, a thing which frequently happens. On the other hand, 
if they are kept well fed and in clean water, such a lot of worms will 
have several divided specimens almost every day, and these pieces 
will often redivide before they have reached normal proportions. 
The amount of division is therefore directly dependent upon the 
amount of food. It can, however, be checked by foul water. 
In feeding the worms upon crushed snails, the water was often 
fouled and could not be replaced as soon as would have been desir¬ 
able by the pond water in which they were kept. In such cases the 
fission ceased, but from six to ten hours after changing the water 
there seemed to be an epidemic of division. My records show that I 
