Dr. Johnson's observations on the genus planaria. 443 
From a neighbouring pond I collected several capsules of 
this species, which I found attached to the under surface of 
the leaves of the ranunculus aquatilis, and found them to pro- 
duce as follows : 
July 26, 16 young from 5 capsules. 
August 3, 25 7 
12, 10— — -3 — 
making an average of three young to each capsule. 
The young, on their immediate exclusion from the egg, are 
of different sizes, and remarkably active. I have sometimes 
seen them after taking food, so stretched, as nearly to double 
their previous size, a convincing proof of the great extent of 
the abdominal cavity. 
A very singular part of the history of these animals, and 
what does not appear yet to have been noticed, is, that they 
have a double mode of perpetuating their species; 1st. by 
capsules, which, as we have seen, contain several young; 
2ndly, by a natural division of the body into two portions, 
the head part reproducing a tail, and the tail a head, in about 
fourteen or more days, depending upon the state of the atmo- 
sphere. 
Fig. 7 and 8 represent the Planaria when about to divide 
to form two distinct animals. On the third day the separa- 
tion of the head from the tail usually takes place, as at Jig. g. 
When undergoing this division, they remain for the most 
part stationary, keeping the head firmly affixed, twisting 
round the tail, from time to time, with a view of lessening its 
adhesion, and thus more readily effecting its disunion. Almost 
immediately after the head is liberated, it is seen to move 
with all the freedom of the unmutilated, perfect animal. The 
