RESPIRATORY ORGANS IN DECAPODOUS CRUSTACEA. 455 
the animal, may or may not be present. The respiratory 
organs would, as usual, be developed at the bases of the swim- 
ming appendages, i. e. on all those of the thoracic segments. 
Here, as in Apus, they do not represent the epipodites, and 
consequently in all normal Schizopod forms we find either the 
epipodite and branchia coexisting (see fig. 5) or the branchia 
alone left, the functionless epipodite having disappeared, or 
possibly, in exceptional cases (Bentheuphausia), having been 
formed at the same time, and so nearly in the same position as 
the gill as almost to have become part of it. 
Most nearly related to the Archischizopod of living forms 
are the Euphausidae, all of which have branchiae attached to 
the bases of their thoracic appendages, and called “podo- 
branchiae.” One of the lowest of these, Thysanopoda (fig. 3), 
has thoracic limbs very closely resembling those of Nebalia 
(fig. 2) only with gills, and with no epipodites. These gills 
are developed from plate-like outgrowths at the base of the 
appendages, which gradually become branched ( Resp . Org. 1), 
while behind each there arises another branch (Resp. Org. 2), 
which rapidily becomes more complex. Thysanopoda is typical 
of most of the Euphausidae, but in one form, Nematoscelis 
(fig. 4), we find that this second branch (Resp. Org. 2), which in 
form exactly resembles that of other Euphausidae, is attached 
not to the same joint of the limb as the first but to the next 
joint (i. e. to the basopodite instead of to the coxopodite) . This, I 
take it, is merely due to the fact that the gill developed while the 
coxopodite and basopodite were undivided, and the two branches 
were carried apart by the separation of these two joints, and 
appear, therefore, as separate branchiae. I have not been able to 
find any actual mention of this being the case, but, on compar- 
ing the limb of such a creature as Thysanoessa or Thysanopoda 
with that of Nematoscelis, no other explanation seems possible ; 
and, considering the small size of the embryo at the stage 
when the gills are developed, the point as to whether the 
two basal joints are united or not at this period may have been 
easily overlooked or not thought worthy of notice. If it is 
once admitted that this change of position is due to the exact 
