528 PROCEEDINGS; BOSTON SOCIETY NATURAL HISTORY. 
seem to be plenty of food and time to accumulate enough material 
behind the pharynx to make a new worm. Because of the latter 
condition, tail pieces redivided much sooner than heads. The short¬ 
est time in which a head redivided (tenth day) was just twice that 
in which tails frequently redivided. 
In the laboratory, the intervals between divisions were so irregu¬ 
lar that nothing can be said except that tail pieces redivide much 
sooner than the corresponding heads. One must suppose that in 
nature some individuals go for a longer time than others without 
division, since the heads and tails when collected are from worms 
of all sizes. 
To show the irregularity between the acts of normal fission, I 
have inserted a text figure which has for its basis the record of three 
individuals which were isolated and followed for 31, 33, and 32 days 
respectively. Beginning with the whole specimens 1, 2, and 3 on 
the left, their acts of normal fission into a head and a tail piece are 
indicated by the divergence of the horizontal lines and the time at 
which each occurred by the scale of days given below. I have sim¬ 
ilar records of quite a number of the products of division for a 
longer or shorter time than the above, but these three cases suffi¬ 
ciently indicate the entire lack of any observed regularity in the 
intervals between the acts of fission. 
JWYS 1 2 3 4 5 6 7 8 9 10 'll 12 13 14 15 -16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 
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