1878.] 



AND DISTRIBUTION OF THE CRAYFISHES. 



759 



more widely separated, and at length disappear, the base of the seta 

 being smooth. A canal traverses the whole length of the seta. 



These bundles of setas were regarded as branchiee by Brandt and 

 Ratzeburg j but they certainly have no direct relation to the respi- 

 ratory function. They may aid in keeping parasites out of the 

 branchial chamber. 



The setae which fringe the base of the podobranchia are straight 

 stiff, hollow, and composed of a cylindrical basal portion, and an 

 acuminated acicular terminal part, which often appears articulated 

 with the foregoing ; the junction of the two is marked by an in- 

 flexion of the wall of the seta. The acicular part (fig. 1, v.) is fringed 

 with minute pointed scales, which, in the setfe of the anterior part 5 of 

 the base, pass into long lateral processes, and give the seta a penni- 

 form appearance. None of these seta: are hooked at the apex. 



The only differences, except those of size, which are observable in 

 the six podobranchiae, are, that the external lobe of the lamina 

 which is not nearly so large as the internal, in the hindermost 



Fig. 2. 



A. The podobranchia (epipodite) of the first maxilhpede of Astama JkmatiUa 

 viewed from the outer side (x 4). B. The podobranchia of the first 

 maxillipede ot Astacoides madagascariensis, outer side ( X 2) : br branchial 

 filaments C. The podobranchia (epipodite) of the first maxillipede of 

 Astacus fluviatihs, viewed from behind ( x 4). D. The podobranchia of 

 the first maxillipede of Parastacus brauliemis, from behind ( X 4) • br 

 branchial filaments. In each figure a marks the pedicle by which the more 

 or less modified podobranchia is attached to the eoxopodite. 



branchia increases in relative size, until, in the most anterior 

 branchia it becomes larger than the internal lobe. The penniform 

 setae are more numerous on the bases of the anterior podobranchias. 

 The first maxillipede is said, and, in a physiological sense, rightly, 

 to possess no branchia ; but it is provided with an appendage (fin-! 

 2, A, c) which is undoubtedly the homologue of the podobran- 

 chia of the other thoracic limbs. This is a soft membranous 



50* 



