318 A HISTORY OF RECENT CRUSTACEA 
three and generally the first five segments of the pleon are 
short, whether fused or articulated ; the sixth comprising 
the telson is usually the largest, and its appendages con- 
stitute the only pair of uropods. The pleopods when 
present are almost always closely overlapping. 
Instead of being contented with the single genus 
Oniscus, as in the first edition of Linnzeus’s ‘System of 
Nature,’ the Isopoda now occupy seven tribes, several of 
which are of great extent. These are named respectively 
Chelifera, Flabellifera, Valvifera, Asellota, Phreatoicidea, 
Epicaridea, and Oniscoidea. 
Tribe 1.—Chelifera. 
This first tribe, the ‘claw-bearing’ Isopods, is a com- 
paratively small one, but it has many peculiarities and is 
controversially interesting. It has to do with narrow, 
subdepressed, or subcylindrical animals, in which the head 
is united with the first or even the first and second seg- 
ments of the pereon. The segmerts of the pleon are 
sometimes fused together, and in that case the pleopods 
are wanting. Otherwise these appendages are generally 
present, and consist of a two-jointed stem and two un- 
jointed branches, but differing from those of the Isopeda 
in general in being swimming rather than branchial 
organs. In this tribe respiration is carried on by means 
of a branchial chamber situated under the sides of the 
carapace to the rear. The first maxille have a backward- 
directed ‘palp’ similar to that described in the Cumacea, 
and no doubt fulfilling the same function of cleansing the 
branchial chamber from obstructions. It is supposed to 
be a part of the endopod turned backwards. The maxilli- 
peds have also a remarkable structure directed backward 
into the branchial chamber. This by its attachment to 
the first joint is recognised as the epipod. By its rhyth- 
mical movement to and fro within the cavity it maintains 
a constant influx of water for the oxygenation of the 
animal's blood. In evident connection with this respira- 
tory process in the front part of the body is the circum- 
