36 Traces of Unity in the Appendicular 



gated into a finger-like process, against which the move- 

 able claw-like metatarsus can be applied after the 

 manner of a thumb. Attached to the base of each 

 " foot " are two appendages, the palp and the flagellum, 

 and also a gill, for in reality the gill is as much a part 

 of the " foot " as the palp and flagellum. In the crab 

 and lobster the four hindermost pairs of " feet " end in 

 single claws and only the first pair is cheliferous, and no 

 other special difference is to be noticed in these or other 

 decapods except this, that in the hermit crab the 

 flagellum is wanting, and the work of this appendage 

 which is that of sweeping over the surface of the gill so 

 as to free it from foreign bodies of any sort is trans- 

 ferred to the fifth pair, the edge of the carapace being 

 raised so as to allow their insertion into the branchial 

 chambers when the work of sweeping has to be done. 

 In the stomapod, isopod, and branchiopod crustaceans 

 the appendages corresponding to the " true feet " of the 

 decapods are much more rudimentary and not readily 

 distinguishable from the other appendages except by 

 their relative position, especially in the two latter orders, 

 these feet, all of them, being six-jointed and ending in 

 simple non-retractile hooks in the isopods, and merely in 

 jointless flattened plates or vesicles, serving for gills, or 

 fin-feet, or marsupial plates for the attachment of ova, 

 one or all, as the case may be, in the branchiopods. In 

 these two orders the parts corresponding to the five 

 pairs of " true feet " in the decapods are very much 

 alike : in the stomapods, as in the Squilla Mantis of the 

 Mediterranean, they differ materially, the first two pairs 

 having modified chelae, with the full number of six 

 joints, and with the forceps differing from that of the 

 decapods in this that the unciform metatarsus, instead 

 of closing upon a finger-like projection from the tarsus 



