INTRODUCTION, XXxlll 
other general of the aberrant type, where it is situated, 
according to the observations of Fritz Miiller, in the dorsal 
surface of the pereion, corresponding in position with that 
of the respiratory systems of the various orders. 
In the Amphipoda, the branchiz are by no means*the 
simple sacs that they have been described. They are 
situated upon the inner surface of the coxe, and assume 
the form of leaf-like hollow plates, ranged in parallel lines 
on each side of the sternum (Fig. 10), and are attached to 
every pair of legs except the first in 
the females, and generally the last 
in males ; though, in Gammarus, we 
have seen the seventh pair fur- 
nished with branchiz as well as the 
preceding. In the Aberrantia, the 
number of sacs is reduced to two 
or three pairs. In this order they 
homologize with the branchiz of 
the decapod type, each branchial 
appendage being viewed in the light 
of a single plate of the compound 
organs of the higher type; or rather, 
perhaps, they bear best comparison 
with the same organ as it appears 
in the larval condition in the 
Brachyura. The great distinction 
in their character is derived mostly 

from the appearance which these 
ice 10" organs assume in the higher forms, 
being that of an internal position. But this is one 
of appearance only. The branchize are overcapped by 
the monstrous production of the cephalic shield m the 
Stalk-eyed orders of Crustacea, a circumstance that gives 
to the portion of the dermal skeleton that it covers the 
C 
