276 



THE CRUSTACEA 



similar transference of the gills from the limb to the body- wall in 

 the development of Cratigon (Caridea), and Bouvier in Uroptychus 

 (Galatheidea). Glaus concludes from these observations that not 

 only the podobranchiae but also the arthro- and pleurobranchiae 

 are originally appendages of the limb. The absorption of the 

 proximal part of the limb into the body-wall is of importance in 

 view of Hansen's recognition of a pre-coxal element in the ap- 

 pendages of various Crustacea. 



The origin of the podobranchiae by differentiation of part of 

 the epipodite is also clearly shown in the development of Penaeus. 

 The most distal of the three outgrowths mentioned above early 

 becomes bilobed. The distal lobe, which lies in front of the 



B. 



FIG. 164. 



Two stages in the development of the branchial system of Penaeus. (After Glaus.) A, early 

 stage; B, later stage after appearance of the rudiments of pleurobranchiae. mxp l -mxp3, 

 maxillipeds ; Z!-Z 5 , legs; a, distal series of rudiments giving rise to mastigobranchiae and 

 podobranchia (on mxp-); b, c, rudiments of arthrobranchiae ; d, rudiments of pleurobranchiae. 

 In B the distal rudiment on mxp% is dividing into podobranchia (pft) and mastigobranchia (mb). 



proximal one, develops in the case of the second maxilliped (Fig. 

 164, B, mxp*) into the podobranch (in the other appendages it 

 disappears), while the proximal and posterior lobe becomes the 

 epipodite or mastigobranchia of the adult, a bilobed membranous 

 lamina which extends upwards into the branchial chamber between 

 the gills. On the first pair of maxillipeds the distal lobe remains 

 simple and persists as the distal part of the laminar epipodite of 

 the adult. It is remarkable, however, that in the only cases in 

 which the epipodite of the first maxilliped develops branchial 

 filaments' (in some Parastacidae), these are borne, not by the distal 

 part which appears to represent the podobranchia, but by the 

 proximal division. 



In most Reptantia the podobranchiae have a similar relation to 

 the mastigobranchiae to that just described in Penaeus. In the 



