7f>8 PRCfF. T. H. HUXLEY ON THE CLASSIFICATION [Jlllie 4, 



truncated anterior end with the coxopodite of one or other of the 

 thoracic limbs, from the second maxillipede to the penultimate am- 

 bulatory limb inclusively. I shall therefore term these gills podo- 

 branchice. A fold of the integument rises over the outer face of the 

 basal portion of the branchia, which is freely movable, inwards and 

 outwards, upon its articulation. 



The basal portion of the branchia curves slightly upwards, and is 

 continued into the comparatively narrow stem (fig. 1, st), which 

 bends up at right angles to the base, and lies nearly parallel with 

 the inner wall of the branchial chamber. Rather beyond the middle 

 of its length, the stem expands into the broad lamina (fig. 1, V), the 

 two lobes of which are folded together like the leaves of a partly 

 open book, their free rounded edges being turned backwards and 

 the uniting fold forwards. The lamina of each branchia, from the 

 second to the sixth, is received between the lobes of the lamina of 

 that which precedes it, and, from the first to the fifth, receives the 

 lamina of the branchia which follows it. 



The edges of the laminae are beset with minute hooked spines, 

 seated upon low tubercles (fig. 1, iv.), and are flat ; but, a short 

 distance within the edge, each lamina is folded longitudinally in such 

 a manner as to give rise to ten or twelve plaits. From the mode of 

 their formation, these plaits or folds are, of course, as distinctly 

 marked on the outer as on the inner faces of the laminje (fig. 1, in.). 

 Small elevations, terminated by minute hooked setae, like those on 

 the edges of the laminae, are observable on both faces, particularly 

 on the ridges. 



The inner lobe of the lamina is continued down the stem as a 

 decurrent gradually narrowing fringe, which terminates on the inner 

 face of the basal portion (fig. 1, n. g). The outer lobe ends more 

 abruptly, in a thin ridge which lies external and posterior to the last. 

 Between this ridge and the fringe there is a longitudinal groove, 

 which occupies the inner and posterior face of the stem. 



A short distance from the upper edge of the lamina, and pro- 

 ceeding, as it were, from the anterior face of the median fold of the 

 latter, the apical plume (fig. 1, i. a p) is given off. At its base this 

 is beset with cylindrical branchial filaments ; but at its free extre- 

 mity it becomes simple and filiform ; at its attached extremity, it 

 passes into the stem of the gill, of which it is seen to be the direct 

 continuation. The whole of the outer and anterior face of the stem, 

 as far as the basal fold, is beset with branchial filaments, of which 

 those in the region of the lamina are the longest, while towards the 

 base they are shorter and more closely set. 



Close to the attachment of the podobranchia, the upper surface of 

 the coxopodite presents a rounded elevation, from which a great 

 number of long, flexible and tortuous setae proceed (fig. 1, vi.). 

 When undisturbed, they ascend vertically between the gills on the 

 sides of the branchial chamber. The apex of each of these coxopo- 

 ditic setae is acute ; and at a short distance from its extremity it is 

 beset with numerous short foliaceous scales, which gives it the aspect 

 of a minute catkin (fig. 1, vn.). Further down these scales become 



