PHYLLOBRANCHI^. 271 



The prawn, in fact, presents us with an extreme case 

 of that kind of modification of the branchial s^^stem, of 

 which PencEus has furnished a less complete example. 

 The series of the podobrancliise is reduced almost to 

 nothing, while the large pleurobranchise are the chief 

 organs of respiration. 



But this is not the onl}^ difference. The prawn's 

 gills are not brush-like, but are foliaceous. They are 

 not tricJiohranchicB, but pliyllohrancliice ; 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 lamellae (fig. 68, C, C, I), 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 downw^ards. These lamellae 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 phyllobranchiae of the prawns are in 

 appearance from the trichobranchise of the preceding 

 Crustacea, they are easily reduced to the same type. For in 

 the genus Axius, 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 



