518 CRUSTACEA MALACOSTRACA. 



connecting the limb with the body.* Though the base of the 

 limb, bearing the proximal branchiae, projects in the larva, it 

 is subsequently merged in the trunk, and hence the two bran- 

 chiae which it bears are attached in the adult to the body wall 

 (pleurobranchiae). The others, attached to the articular mem- 

 brane and the basal segment of the adult limb, become the 

 arthro- and podobranchia respectively. "[* 



It would thus appear that the branchiae of Decapods were 

 all originally appendages of the limbs, as are those of Anaspides, 

 and that their partial change in relation is due to the fusion of 

 the proximal segments of the limbs with the trunk possibly 

 owing to the formation of a branchial cavity. The distal 

 branchial outgrowth (podobranchia) may develop into a simple 

 plate, or into a plate bearing branchial filaments, as in Astacus, 

 or into a plate bearing branchial filaments, but also extended 

 into a process free from such filaments. Although it is probable 

 that all the branchiae are epipodial in nature, the terms epipodite 

 and flagellum are, in Decapod terminology, applied to the more 

 or less tapering plate-like process of the podobranchia, which 

 bears no branchial filaments. It is the mastigobranch of Spence 

 Bates' nomenclature. Huxley uses the term lamina for a 

 plate-like part of the podobranchia which bears gill filaments, as 

 e.g. in Astacus. 



The number of branchiae varies greatly and is not constant 

 even in the same genus. The slender pelagic Leucifer is the only 

 known Decapod without them. The number is generally largest 

 in the Macrura Reptantia (Palinurus and Astacopsis 21, Huxley, 

 Astacus 18, Huxley ){ and smallest in the Caridea (Pandalus 12, 

 Crangon 6, Glaus) and Brachyura (Cancer 9, Huxley, Pinnotheres 

 3, Glaus). Among the Penaeidea, Penaeus has 18, Glaus and 



* The question how the four branchiae of the limbs of Decapods are 

 related to the two on those of Anaspides appears too far from solution for 

 profitable discussion here, cf. Borradaile, I.e. pp. 461-463. 



f Huxley concluded from his examination of the Crayfish that the 

 branchial equipment of a segment was 1 podobranchiae, 2 arthro branchiae 

 and 1 pleurobranchiae. But Glaus, from his study of the development of 

 Penaeus is inclined to regard the posterior arthrobranch of Huxley as be- 

 longing to the proximal series, i.e. as a pleurobranch. 



J It has recently been shown by Miss M. Moseley that a rudimentary 

 posterior arthrobranch (a second pleurobranch on Glaus' reckoning), at the 

 base of the first maxilliped, is present in Astacus ftuviatilis Fabr. (sensu 

 stricto), which would bring the number of gills for this species to 19. See 

 Q.J.M.S. Vol. 48 (1905), p. 359. 



