60 Mr. J. Wood-Mason on Scolopendrella. 
picion the correctness of which has been verified by the study 
of several stages in the postembryonic development of the 
animal. So that eleven pairs of rudimentary legs and eleven 
pairs of functional ones, or twenty-two in all, marking as 
many separate somites, are to be made out in this animal 
between the head and the tail. 
The presence of the two apodous terga and of the eleven 
pairs of rudimentary feet seems intelligible only on the suppo- 
sition that Scolopendrella has been evolved from a form with 
twenty-two distinct and complete leg-bearing somites, by the 
reduction to rudiments of the legs (accompanied by the abor- 
tion of the metamerically arranged organs, such as stigmata 
and excretory pouches of the somites), and the suppression of 
all but two of the terga of alternate somites. 
The terminal tergum, which is longer than broad, truncated 
at both ends, and slightly arched at the sides, probably con- 
sists of two connate terga. It is converted posteriorly, appa- 
rently by the inbending of its sides, into a complete rmg 
divided by vertical partitions into three compartments, to the 
two outer of which the perforated caudal appendages are at- 
tached, and into the median and dorsally emarginate one of 
which it is probable that the anus opens. 
Between the complete ring formed by the posterior end of 
the last tergum and the last of the series of double sterna 
are interposed two plates, which I take to be the sterna of the 
last somite; of these the posterior, which extends beyond the 
extremity of the body, is soft and deeply cleft im the middle 
line; while the anterior, which is semicircular, and covers like 
an operculum all but the free margin of the posterior, is firml 
chitinized and has its straight hinder margin entire. The 
ostero-lateral margins of the latter are each produced into a 
short cylindrical process encircled with setee and hollowed out 
at its extremity into a cup-like concavity, from a tubercle in 
the bottom of which springs an excessively long and fine and 
gradually tapering simple seta; this pair of setigerous pro- 
cesses, which have much more the appearance of rudimentary 
legs than of mere processes, especially in the larva, are pro- 
bably sensory organs of some kind ; whilst the aperture of the 
genital organs (which, according to Menge, to whom we are 
indebted for all our knowledge of the internal anatomy, open 
at the hinder end of the body) is probably situated in the 
former rather than in the anterior part of the body, where 
I have hitherto failed to make out any other openings but 
those I have described below. 
Organs of Respiration.—These consist of eleven tracheal 
arches, opening by as many pairs of minute pores, situate on 
—9 
