296 A HISTORY OF RECENT CRUSTACEA 
maxilliped is not of exceptional structure. The second 
joint is large, usually with a terminal plate having 
coupling hooks on its inner margin. The seventh joint is 
wanting, and in the genus Campylaspis the fifth and sixth 
also, the fourth joint being rudimentary and terminal, and 
the second joint in this case not carrying an apical plate. 
All the known forms, however, agree in having in con- 
nection with the first maxillipeds a remarkable respiratory 
apparatus. The hinder part of this, being certainly at- 
tached to the first joint, no doubt represents the epipod, 
while the front part, though coalesced with the hinder 
part, may in reality be derived from the second joint, and 
in that case it will, as Sars supposes, represent the exopod. 
This latter opinion is sometimes contested, but, however 
that may be, it is to Sars that belongs the honour of having 
first clearly made known the manner of action of these 
structures, as well as of having beautifully delineated the 
intricate forms which some of them assume. The epipod 
directed backwards into the branchial cavity finds many 
parallels, asinthe Myside, where likewise its oscillations con- 
tribute to maintain a steadily renewed flowing in of water. 
But in the Cumacea the epipod is usually furnished with 
branchial sacs, sometimes in large numbers and spiraily 
arranged. ‘To this there is nothing properly parallel in these 
appendages elsewhere. ‘The anterior or exopodal portion 
of the apparatus is even more singular. The pair of these 
plates on the two maxillipeds are sometimes firmly coalesced 
together, but in any case they meet and form a tube which 
can be projected in front of the head and again withdrawn. 
As long as the annnal continues the peculiar twisting 
movements which in life it so frequently exhibits, the 
branchial apparatus is completely motionless. But when 
it is in turn quiescent, the transparent end of the tube is 
shot out under the pseudo-rostral end of the carapace 
between the bases of the antennee, while the gill-bearing 
epipodal plates are set oscillating. When the gill-plates 
move forward the water is forcibly expelled through the 
end of the tube. When the plates move back, the slit- 
like orifice of the tube closes of itself through the elasti- 
