RESPIRATORY ORGANS IN DEOAPODOUS CRUSTACEA. 463 
left alone on the basal joint of the leg. In which, although 
very peculiar, appears to be another form, very closely allied 
to Penseus (Cerataspis), the podobranchs go on developing 
with the rest, and are present in the adult on nearly all the 
thoracic segments (vii-xii) as well as, and very close to, the 
epipodites . 1 It is therefore probable that Penseus, in the course 
of phylogenetic development, has lost its podobranchs. This 
loss, I think, can be explained by the fact that Penseus hatches 
at an earlier period than Cerataspis and other Decapods, for in 
comparing the different gill formulae of the Decapods, espe- 
cially those mentioned in Professor Huxley’s book on the 
Crayfish, we notice that it is in forms in which, like Astacus, 
the young is hatched only when fully developed, that the podo- 
branchs are the most fully and the pleurobranchs the least 
fully developed. 
Thus we see that in Astacus and in all the Astacidae very 
nearly the full number of podobranchs is present, while the 
number of pleurobranchs varies, there sometimes being none 
at all, sometimes one or two rudimentary ones and one well- 
developed one as in Astacus. 
In Homarus, and those Decapoda macrura whose young 
are hatched rather earlier than in Astacus, more pleurobranchs 
are developed. Penseus, which has lost its podobranchs, and is 
well supplied with pleurobranchs, is, you will remember, the 
only Decapod which hatches in the nauplius or earliest larval 
form. This at first seems not to be in accordance with what we 
find in the only other Malacostracan forms which hatch in the 
nauplius condition, namely, the Euphausiidse, where, as we have 
already seen, the gill is always attached to the base of the limb, 
and is truly podobranchial, though in the other Schizopods (Lo- 
phogastridse), which hatch at a later stage, these have moved to 
the arthrodial membrane. But when we take into account the 
more delicate structure of the gill of these higher Malacostra- 
cans, and the consequent need of protection, this want of 
agreement can, I think, be fully explained. From the ancestor 
1 See Dolirn “Untersuchungen iiber Bau und Entwickelung der Arthro- 
poden,” ‘ Zeitsclir. f. wiss. Zool.,’ vol. xxi, 1871 (fig. 32). 
