52(3 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
in the tailpiece (pi. 9, fig. 3-7, 13, 14, 16-18) this forms the part 
of the new head lying anterior to the tips of the auricles. In the 
head piece it merely caps the posterior end (pi. 10, figs. 19, 28, 29). 
During the second day this new tissue increases in size. On the 
third day, the first indication of the eyes is found as two minute 
dots just on the line between the old and the new tissues (pi. 9, 
figs. 4, 14, 16). The halo about them is first seen when the pigment 
flecks become apparent on the fourth or fifth day (pi. 9, figs. 6,18). 
The area of the new tissue has by the fourth day become consider¬ 
ably larger and its anterior edges have begun to make the angles of 
the normal head (pi. 9, figs. 5, 17). The points of the auricles do 
not show plainly until the sixth or seventh day (pi. 9, figs. 7, 8) and 
even on the tenth day, although the head has reached its normal 
shape, it may still be much lighter in color and so indicate the 
recent fission. From the sixth day on, it is difficult to distinguish 
a tail piece from a normal specimen and many tail pieces even of 
this age are probably counted as whole specimens in collecting (pi. 
9, figs. 7, 8). 
A word explaining some of the figures will review these changes. 
Figure 2 to 7 of plate 9 represent the changes of a single tail piece 
on six successive days, and figures 20 and 21 of plate 10 represent 
this piece redivided on the eighth day. Figure 16 to 18 (plate 9) 
represent a single tail piece on the third, fourth, and fifth days respec¬ 
tively after normal fission, and figures 13 and 14 (plate 9) another 
tail piece on the second and fourth days. Figure 8 (plate 9) is a tail 
piece the eighth day after separation. All the figures of the external 
changes were made from live specimens studied under a dissecting 
microscope. They are as accurate as the constant movement of the 
extended animal will allow and were drawn only after careful 
measurements of the length, breadth, and other proportions with a 
pair of compasses. For the sake of clearness the finer branches of 
the gut are omitted. 
When the tail is pinched off, there has been no rearrangement of 
the gut and we therefore have two parallel gut rami with their vari¬ 
ous branches (pi. 9, fig. 2; pi. 10, fig. 21; pi. 11, fig. 25). The gut 
in whole worms of this species has, however, so many branches and 
secondary connections that it is exceptional to find but two long 
rami in the posterior end (pi. 10, figs. 22, 23, 29 ; pi. 11, fig. 31). 
Three or four is the common number and there are sometimes as 
