RESPIRATORY ORGANS IN DEOAPODOUS CRUSTACEA. 453 
gite took place after or before this respiratory fold was 
established. 
The primitive position, therefore, of the respiratory organ is 
behind the swimming organ. We shall see that this is the 
case in the Schizopods, Stomapods, and also in the higher 
group of the Isopods. The Archimalacostraca, however, which 
developed from the same source as the Phyllopods (i. e. from 
an Archi-entomostracous form), probably had not a settled 
respiratory organ ; this might or might not be present. For 
we find in Nebalia, which of all living forms most nearly repre- 
sents the Malacostracan ancestor, that there are no special 
branchial organs. The epipodites (fig. 2) of the thoracic 
limbs, as in the Entomostraca, and as in no other Mala- 
costracan forms, are still used for swimming, and probably 
move rapidly enough to keep the surface of the body sufficiently 
well supplied with oxygen. 
While the genus Nebalia has been modified but slightly 
from its original form, another form, very closely allied to it, 
and having probably behind its swimming epipoditic plates 
plates or folds of the skin for branchial purposes, has been 
subject to more severe competition, and has become changed 
and modified in many different ways. This form probably 
had the typical Malacostracan number of segments and appen- 
dages (nineteen and a telson, the telson being possibly divided 
externally, so as to give the appearance of either one or two 
extra segments, as in Nebalia), and an elongated heart. From 
this a more stable form has developed, which we may call the 
“ Archischizopod,” not, however, until it had given rise to a 
form which became the ancestor of the Stomapoda. The 
Archischizopod, when formed, would have acquired a fixed 
number of segments, and would differ chiefly from the Archi- 
malacostraca in its different mode of swimming; for it is no 
longer the epipoditic plate which is the swimming organ, but 
the other primitive branches of the Crustacean limb, and espe- 
cially the exopodite, which is developed and modified for this 
purpose. Consequently the epipodite, having lost its primitive 
signification, and at this stage being of no special advantage to 
