22 ALFRED GIBBS BOURNE. 
a slightly further advanced condition than the segment be- 
hind it. 
I have studied the order of development of the embryonic 
setee in a very large number of embryos of P. pellucida, and 
figs. 26—28 are given as three typical stages ; they are diagram- 
matic, but accurate in respect of the number of sete present. 
The embryo is supposed to have been slit open and flattened, 
and all the sete on one side of the body areshown. V. V. and 
D. D. represent the median, ventral, and dorsal lines. 
These figures show that the sete develop, asa rule, in couples, 
that the most ventral couple is the earliest to develop, that the 
couple which become the most dorsal follow next, and that 
then with considerable regularity ventral and dorsal couples 
appear alternately; further that from a very early stage the 
appearance of the sete becomes retarded in Segment 11, a 
retardation which afterwards extends to the next two or three 
segments (in stages later than those figured). 
All the sete in figs. 26—28 are embryonic sete, no perma- 
nent having yet appeared even in fig. 28. 
In Mahbenus and in Perionyx the order in which the sete 
appear is different; the most ventral appear first, then those next 
to them, and so on, the most dorsal appearing last. I expect 
that this represents a more modified condition; in P. pellucida 
each segment passes in the condition of its sete through a 
stage which remains as the permanent condition in an octo- 
cheetous form. 
In Moniligaster, Acanthodrilus, and Lumbricus the earliest 
seta to develop on each side in each segment is the seta which 
becomes seta ] (i.e. the most ventral), the next to develop is 
seta 4, the next seta 2, and then seta 3. 
Adult Condition and Formation of the Permanent 
Setz.—The sete in the adult P. pellucida in any segment 
lie at irregular intervals from one another, and the number of 
the sete in a ring varies to a small extent. The actual num- 
ber of sete in a particular specimen lying on one side of the 
body in Segments xxv—xxvi1 are shown in fig. 29, the relative 
distance between one seta and another is accurately shown, 
