794 On the Homologies of the Crustacean Limb. [October, 



homologous with the exopodite and endopodite of the Decapoda. 

 In the latter the maxillipede is highly differentiated; in the tho- 

 racic limbs of the Phyllocarida and Merostomata it is uniaxial 

 and jointed, but in the Phyllopoda not truly jointed. In the sim- 

 plest Decapod limb, that of the abdomen, we have a stem suc- 

 ceeded by two divisions, the exopodite and endopodite ; in the 

 thoracic feet we have but one of these branches, the endopodite, 

 while in the maxillipedes, the most differentiated, we again have 

 a stem and two branches (endopodite and exopodite), together 

 with the gill and flabellum. Thus the entire leg of th'e Phyllo- 

 pod (without the gill and flabellum) is homologous with the endo- 

 podite of the Decapod maxillipede, and the gill and flabellum 

 with those of the Decapoda. 



Camparis ! iiubs of Nebalia (Phyllocarida). — 



Not to enter into detail, by a glance at Plate x and the figures in 

 Plate xiv, it will be seen that the thoracic appendages of Nebalia 

 consist of an inner axial-jointed portion (the endopodite), which 

 may perhaps be regarded as homologous with the endopodite of 

 the Decapod maxillipede, and also with the thoracic legs of the 

 lobster. This also corresponds to the endopodal unjointed por- 

 tion of the Phyllopod thoracic limb. In the exopodal or respira- 

 tory portion (ex) the upper part corresponds to the Phyllopod 

 gill, and the double lower portion to the flabellum. 



Comparison with the feet of Limulus (Merostomata).— The resem- 

 blance between the abdominal legs of Limulus and the thoracic 

 ones of Nebalia is apparent on inspection of PI. x, figs. 3 and 4- In 

 Limulus the shell flares out widely and the appendages are united 

 in the middle, although separate in embryonic life, so that this is 

 a feature of secondary importance. The point of special interest 

 is that the abdominal feet of Limulus may, as in the thoracic 

 appendages of the Phyllopoda and of the Phyllocarida, or the 

 maxillae, maxillipedes, and thoracic feet of the Decapoda, be 

 divided into an inner endopodal portion (whether ambulatory or 

 natatory), and an outer or respiratory portion, as in Nebalia and 

 Decapoda. The endopodite of Limulus (en) is axially-jointed, 

 there being three well marked joints to this part of the limb. 

 The branchiate portion of the limb (ex) is Homologous with that 

 of Nebalia, and the epipodital or branchiate portion of the Deca- 

 pod thoracic limb. At the same time that of Limulus presents 

 some remarkable peculiarities, /. e., the exopodal (or epipodital) 



