320 A HISTORY OF RECENT CRUSTACEA 
flattened spines being of use for specific distinctions. One 
or both of the pairs have usually small two-jointed exopods. 
The perzeopods are slender, with many sete and spines. 
The pleopods have two long setiferous branches, and the 
uropods two that are many-jointed and filiform, the mner 
elongate. 
The small exopods above mentioned, when seen only 
in cabinet specimens, might be considered rudimentary or 
of little importance. But this would not be a right way 
of regarding them. The branchial chamber has two little 
openings, one for the entrance, the other for the exit of 
the water used in breathing. In front of the latter of 
these orifices in the living animal, M. Yves Delage points 
out that the exopods of the first gnathopods with their 
terminal setee vibrate so rapidly that the machinery can 
with difficulty be seen, while those of the second gnatho~ 
pods behave in the same manner in front of the entrance 
apertures. By this contrivance a lively current of water 
is introduced to the branchial chamber, and, when its 
virtue has been exhausted, the parting guest is speeded to 
a convenient distance. When there is only one pair of 
exopods, it is those at the gate of egress which remain, 
and it is no doubt of less importance that the entering 
current should be rapid than that the used-up water 
should be effectively dismissed. 
Five genera are at present assigned to this family. 
The ocular lobes are not movable, as they were at one 
time supposed to be. 
Apseudes, Leach, 1814, has six segments of the trunk 
free, a well-developed scale on the second antennee, a well- 
developed ‘ palp’ on the mandibles, exopods on both pairs 
of gnathopods, and five pairs of pleopods, with both 
branches usually one-jointed. 
Parapseudes, Sars, 1880, has six segments of the trunk 
free, the antennal scale rudimentary, the mandibular 
‘palp’ very small, exopods on both pairs of gnathopods, 
and only four pairs of pleopods, which have one of the 
branches two-jointed. The single species is Parapseudes 
latifrons (Grube) from the Mediterranean. It was at one 
