HOMOLOGIES OF THE CRUSTACEAN LIMB. 



401 



ferentiated from tlie endopodal portion of the limb. But when we 

 look at the tliird pair of limbs of the female of the same Cladoceran 

 (fig. 28), we find "an epipodal portion (flabellnm [ex. I and gill) differ- 

 entiated from the endopodal portion of the limbs. The endopodal 



_• - „ end 



..^<,^^^^^ST^ 



\ W 



Pig. 28. — One of the third pair of limhs of Moina: end, the endopodal portion ; ex, 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 

 tbeuce 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 limhs* — 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 thora<;ic api^eudages 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 protopodit^, which is wanting 

 in the Phylloi)oda and all lower Crustacea, with no endital lobes as iu 

 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 Decapods have probably come down to ns 

 by a different branch of the Crustacean ancestral tree, and have arisen 

 entirely independently of the Phyllopodous branch, by a line leading 



* In comparison with those of the Nebalia, the reader is referred to the last chapter 

 ou PhyllocarUia, 



