PHYLLOBRANCHIA, 271 
The prawn, in fact, presents us with an extreme case 
of that kind of modification of the branchial system, of 
which Peneus has furnished a less complete example. 
The series of the podobranchie is reduced almost to 
nothing, while the large pleurobranchie are the chief 
organs of respiration. 
But this is not the only difference. The prawn’s 
gills are not brush-like, but are foliaceous. They are 
not trichobranchia, but phyllobranchie; that is to say, 
the central stem of the branchia, instead of being beset 
with numerous series of slender filaments, bears only two 
rows of broad flat lamelle (fig. 68, C, C’, 1), which are 
attached to opposite sides of the stem (C’, s), and gradu- 
ally diminish in size from the region of the stem by which 
it is fixed, upwards and downwards. These lamelle are 
superimposed closely upon one another, like the leaves of 
a book; and the blood traversing the numerous passages 
by which their substance is excavated, comes into close 
relation with the currents of aerated water, which are 
driven between the branchial leaflets by a respiratory 
mechanism of the same nature as that of the crayfish. 
Different as these phyllobranchie of the prawns are in 
appearance from the trichobranchie of the preceding 
Crustacea, they are easily reduced to the same type. For in 
the genus Aius, which is closely allied to the lobsters, 
each branchial stem bears a single series of filaments on its 
opposite sides; and if these biserial filaments are sup- 
posed to widen out into broad leaflets, the transition from 
