520 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
striction appears a short distance behind the mouth and a new 
worm is produced at the posterior end of an old one. Before the 
two separate, the new individual has begun to feed for itself and its 
eyes, brain, gut, etc., are in the same condition as those of any 
normal adult specimen. As the time for separation approaches, a 
furrow makes its appearance at the place of division and deepens 
slowly as the internal changes progress. There is little difficulty in 
recognizing individuals about to divide, as for some hours before 
the separation we have two perfect Avorms with the head of the 
smaller attached to the tail of the larger by a narrow isthmus. 
This type is the same as is found in all Rhabdocoeles. 
2. In Planaria suhtentacidata (Zacharias, ’86) the separation 
occurs when the new organs of the tail piece are partly formed. 
The posterior gut rami are fused in front of the new pharynx, but 
the formation of the head has been delayed. There is an external 
furroAV which is the first sign of the approaching fission and is 
visible until the separation occurs four days later. My figure of a 
Planaria maculata on the third day after normal fission (pi. 9, figs. 
5, 17) shows the condition of a tail piece of P. subtentacidata imme¬ 
diately after fission. 
3. The division in P. maculata is at a corresponding place behind 
the pharynx, but is preceded by no external furrow, internal devel¬ 
opment of organs, or histological changes along the line of separation, 
so far as I can discover (pi. 10, fig. 22). The animal pinches itself 
in two at a definite place (pi. 11, figs. 24, 25), and the head and tail 
pieces after division are as though cut in tAvo Avith a knife a short 
distance behind the pharynx. The occurrence of the division at a 
corresponding place links this with (1) and (2), while the separa¬ 
tion without any previous rearrangement of old, or development 
of neAV organs makes the fission of P. maculata almost the same as 
the last. 
4. This is found in many land j^lanarians (Lehnert, ’91 ; von 
Graff, ’99 ; Bergendal, ’87; Fletcher & Hamilton, ’87) Avhen one or 
more pieces of varying lengths are pinched off at the posterior end 
and each fragment, on regenerating the necessary organs, becomes a 
normal individual. • , 
Although Planaria alhissima (Sekera, ’88), in Avhich the division 
occurs just anterior to the pharynx, might be used to fill the place 
(3) Avhich I have assigned to Planaria macidata, the latter com- 
