HOMOLOGIES OF THE CRUSTACEAN LIMB. 405 
ferentiated from the endopodal portion of the limb. But when we 
look at the third pair of limbs of the female of the same Cladoceran 
(fig. 28), we find an epipodal portion (flabellum [ex.| and gill) differ- 
entiated from the endopodal portion of the limbs. The endopodal 
Fic. 28.—One of the third pairof limbs of Moina: end, the endopodal portion; ea, the exopodal (epi- 
podal) portion of the limb. 
portion in the Cladocera is not differentiated, not forming a number of 
well-marked lobes or endites, as in the Phyllopoda, this differentiation 
into six endopodal lobes being peculiar to the Phyllopoda. 
The Cladocerous limb is intermediate in form and complication between 
the Phyllopodous and Ostracodous limbs, and the latter are evidently 
derived from the Copepods, so that there is a continuous ascending 
series from the Copepoda through the Ostracoda to the Cladocera, and 
thence to the Phyllopoda. Hence, as the young of the Copepoda are 
all Nauplii, and also those of the Phyllopoda, it follows that the ances- 
tral form of all the Entomostracous Crustacea, as originally insisted on 
by Fritz Miiller (Fiir Darwin), was a nauplius like animal. 
Comparison with the Decapodous limbs.*—Having studied the homolo- 
gies of the Phyllopodous limbs among themselves, and also compared 
them with those of the Cladocera and Ostracodes, it remains now to 
compare the thoracic appendages of the Phyllopods with those of the 
adult Deeapoda. At the outset, however, it seems nearly impossible to 
compare the swimming legs of the Phyllopods with the abdominal and 
thoracic appendages of Decapods. The thoracic Decapodous legs are 
axially jointed, consisting of an axis or protopodite, which is wanting 
in the Phyllopoda and all lower Crustacea, with no endital lobes as in 
Phyllopods, though the gill and flabellum of the Phyllopods are homolo- 
gous with the gills and flabellum of the Decapod. There is no such 
relation or close resemblance as to lead us to infer that as regards the 
nature of the thoracic and abdominal feet the Decapods have descended 
from the Phyllopods. The Deeapods have probably come down to us 
by a different branch of the Crustacean ancestral tree, and have arisen 
entirely independently of the Phyllopodous branch, by a line leading 
“*TIn comparison with those of the Nebalia, the reader is referred to the last chapter 
on Phyllocarida. 
