RESPIRATORY ORGANS IN DECAPODOUS CRUSTACEA. 461 
. The position of the different branchiae with regard to each 
other in the same segment tends to vary very greatly. 
Huxley has classed the different kinds of branchiae, in com- 
paring them with those of the typical Astacus, as podobranchs, 
anterior and posterior arthrobranchs and pleurobranchs. 
Claus, however, has shown that these names do not, in all 
cases, apply, but that, owing to the undefined limits of the 
arthrodial membrane, they should vary in the different 
families if they are to be strictly correct. Thus what Huxley 
calls “ posterior arthrobranchs ” he calls “ anterior pleuro- 
branchs •” since in Penseus, which is probably a more ancestral 
form than Astacus, the third branchiae (fig. 6, c) are attached 
to the body wall and not to the arthrodial membrane in the 
adult. Claus holds that in the ancestral Decapod the distal 
branchia was, as it still is, a true podobranch. The middle 
one was probably also a podobranch, which in the shortening 
of the coxopodite has moved to the arthrodial membrane ; 
while the two proximal ones were attached near the base of 
the appendage either to the membrane or to the body wall. 
Claus also explains how an arthrobranch may become a pleuro- 
branch by the moving of the arthrodial membrane away from 
the body wall along the limb, so that the proximal portion of 
the membrane may become part of the pleural wall. That a 
podobranch may become an arthrobranch is shown by the 
condition in the larva of one of the Brachyurous Decapods — 
Acanthocaris (fig. 8), as well as in that of Pengeus (fig. 6, a 
and (3 ), where the second gill ( b ) is developed on the basal 
portion of the limb, and only afterwards becomes moved 
backwards to the arthrodial membrane. All such variations 
as these seem to point to an earlier approximation in position 
of the gills, so that from a compound four-fold gill, not unlike 
that of one of the Lophogastridge (fig. 5), the various gills of 
the Decapod may be derived. 
The structure of the gill itself may also, as shown by Claus, 
be derived from that of a Schizopod, and both the typical 
forms of gill observed in Decapods may be so derived. The 
Crayfish and the greater number of long-tailed Decapods 
VOL. XXIX, PART 4. NEW SER. H H 
