[Illustration: FIG. 71. _Palæmon jamaicensis_ (about 5/7 nat. size). A, female; B, fifth thoracic appendage of male.] None of these differences from the crayfish, however, is so great, as to prepare us for the remarkable change observable in the respiratory organs. The total number of the gills is only eight. Of these, five are large pleurobranchiæ, attached to the epimera of the five hinder thoracic somites; two are arthrobranchiæ, fixed to the interarticular membrane of the external maxillipede; and one, which is the only complete podobranchia, belongs to the second maxillipede. The podobranchiæ of the first and third maxillipedes are represented only by small epipodites. The branchial formula therefore is:— Somites and Arthrobranchiæ. their Podobranchiæ. /───────^───────\ Pleuro- Appendages. Anterior. Posterior. branchiæ. VII. 0 (ep.) 0 0 0 = 0 (ep.) VIII. 1 0 0 0 = 1 IX. 0 (ep.) 1 1 0 = 2 (ep.) X. 0 0 0 1 = 1 XI. 0 0 0 1 = 1 XII. 0 0 0 1 = 1 XIII. 0 0 0 1 = 1 XIV. 0 0 0 1 = 1 ── ── ── ── ── 1 + 2 ep. + 1 + 1 + 5 = 8 + 2 ep. {271} The prawn, in fact, presents us with an extreme case of that kind of modification of the branchial system, of which _Penæus_ has furnished a less complete example. The series of the podobranchiæ is reduced almost to nothing, while the large pleurobranchiæ 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 _trichobranchiæ_, but _phyllobranchiæ_; 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 lamellæ (fig. 68, C, C′, _l_), which are attached to opposite sides of the stem (C′, _s_), and gradually diminish in size from the region of the stem by which it is fixed, upwards and downwards. These lamellæ 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.