CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 525 
other himters than their own kind. Another partial explanation of 
the smaller number of tail pieces in my count is, that they become 
almost indistinguishable from whole worms after the sixth or seventh 
day, while in head pieces the proximity of the pharynx to the 
posterior end remains for ten or twelve days as a sure evidence of 
recent division (pi. 10, fig. 29; pi. 11, fig. 30). The abundance of 
tail pieces in all stages of regeneration is sufficient proof that a 
large proportion of them do become perfect worms. There is no 
limit of growth which when reached causes the fission, since the 
heads and tails when collected are of all sizes (compare pi. 10, figs. 
19, 20, pi. 11, fig. 24, with pi. 9, figs. 2, 18, 16). 
The figures on plates 9, 10, 11 which illustrate the following 
description of the external changes were all made fi-oni living speci¬ 
mens, which were kept well fed upon a darkly pigmented pond snail. 
The presence of this food in the digestive tract enabled me to fol¬ 
low the changes of the gut b}^ measurements of the same specimen 
on successive days. In the figures I have only represented the 
main gut branches, as the smaller ones could not be accurately made 
out in the live worm and if shown would only obscure the course of 
the larger ones. 
After their normal separation the heads and tails follow the same 
course as though artificially cut in two (pi. 9, fig. 2-7, 16-18; pi. 
10, figs. 19, 28, 29; pi. 11, fig. 30). I have no observations upon 
any differences between the processes following noi-mal and those 
following artificial separation. Viewed from the dorsal or ventral 
side the naked end of either piece immediately after division shows 
a straight transverse edge with just a little line of white parenchyma 
protruding beyond it (pi. 9, fig. 2 ; pi. 11, figs. 24, 25). Viewed head 
on, the scar is like a transverse section of the worm somewhat 
reduced in size. The sides of the body are curved in slightly as 
though the scar were always kept a little drawn together. If the 
pieces contract, as they frequently do, a crescentic area of scar tissue 
appears, and just behind it a very dark line j^roduced by the draw¬ 
ing together of the 2 :)igment flecks (pi. 9, fig. 1; pi. 10, figs. 21, 26; 
pi. 11, fig. 24). Sections show the joarenchyma to be naked and 
occasionally portions of the gut are in contact with the outside 
(pi. 13, fig. 35). 
During the first twenty-four hours the cut end becomes pointed 
by the appearance of a small triangular area of white scar tissue, and 
