I882.J On the Homologies of the Crustacean Limb. 795 



portion is jointed ; and the gill, instead of being a simple, fan-like 

 extension, as in the Phyllopoda and Phyllocarida, is replaced by 

 a number of flat, thin gill-plates, arranged parallel to each other, 

 in an antero-posterior sense. When, however, we compare the 

 gill, or rather the epipodital portion of the leg of Limulus, with 

 that of the lobster, we have the various fundamental elements, 

 i. e. an artery and a vein passing into the foot and in connection 

 with a number of gill-plates. In the lobster we have along the 

 base of the gill (Fig. 9), collective veins and an artery into which 

 the blood passes after being aerated in a large number of cylin- 

 drical gill-filaments. Morphologically there is a fundamental re- 

 semblance between the two types of branchiae; in Limulus there 

 are gill-plates, in Decapods gill-filaments, each presenting in the 

 aggregate a large respiratory surface. The gills of the Isopoda are 

 in some degree intermediate between the Decapods and the 

 Merostomata. 



When we compare the anterior or cephalic appendages with 

 the thoracic appendages of the lobster, there is a close resem- 

 blance in the axially-jointed endopodite (Fig. 10, end) of Limulus 

 with its large terminal claw to the foot of the Decapod. The 

 absence of the gill or branchiate (epipodital) portion in Limulus 

 is correlated with the ambulatory nature of its anterior or ceph- 

 alic appendages. 



In the trilobites, however, as may be seen by Mr. Walcott's 

 able restoration (Fig. 12), we have attached to the thoracic ambu- 

 latory feet a respiratory epipodital portion. In some respects, 

 then, in the "trilobites we have a style of structure intermediate 

 between the Merostomata and the Decapoda. 



In the trilobite we apparently have, besides a true-jointed loco- 

 motive endopodite (Fig. 12, en), an inner jointed appendage («*'). 

 which may be homologized with the exopodite of the Decapod 

 maxillipcde (Fig. 9). From near its base arises the two singular 

 spiral gills, which are unique. It is to be observed that the two 

 jointed appendages and the stem of the gills arise from what 

 appears to be a true coxopodite, and that this coxopodite is appa- 

 rently homologous with that of Limulus (Fig. 10). It thus ap- 

 pears that a study of the general internal anatomy and of the 

 appendages of the normal, recent Crustacea {Neocarida) throws 

 light upon the structure of the archaic Crustacea (Paheocaridcs), 

 and that the most archaic Neocarida, the Phyllocarida (Nebalia), 



