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Show 784 PROF. T. H. HUXLEY ON THE CLASSIFICATION [JlUie 4, are fixed in pairs by a common pedicle to the arthrodial membrane of the appendages of the tenth and ninth somites-that is, the great forceps and the third maxillipede. The most anterior gill, slenderer than the others, is attached to the arthrodial membrane of the second maxillipede. There are therefore five arthrobranchia?. The podobranchia of the first maxillipede is represented only by the large curved epipodite, which sweeps over the surface of the arthrobranchiae and the pleurobranchia?. The podobranchia of the second maxillipede is divided into a branchial plume, which lies horizontally under the bases of the four anterior arthrobranchia?, and an epipodite, which ascends between the arthrobranchia of its somite and those of the next, and lies internal to the latter, close to the inner wall of the branchial cavity. In the third maxillipede the epipodite is very long, and forms the valve to the afferent aperture of the branchial cavity. Attached to its base is a short truncated branchia, which fits in between the bases of the second and the third arthrobranchia?. The Branchial formula of Cancer pagurus. Somites and their appendages. VII VIII IX, X XI XII XIII XIV Podobranchise. 0(ep) 1 1 0 0 0 0 0 Arthrobranchia?. Anterior. 0 1 1 1 0 0 0 0 Posterior. 0 0 1 1 0 0 0 0 Pleurobranchia?. 0 = 0 = 0 0 1 1 0 0 = = = = ^s ^ 0(ep) 2 3 2 1 1 0 0 2+ ep + 3 + 2 + 2 = 9+ ep. A considerable reduction of the branchial apparatus occurs in some Brachyura, especially the Catometopa. Thus, in Gelasimus the hinder pleurobranchia and the most anterior arthrobranchia have disappeared, and the two podobranchiae are so small as to be almost rudimentary. In the Raninida? and in Latreillia, the Brachyurous metope is incompletely formed; but the branchial system is essentially that of the true Crabs. The same may be said of Dromia, although the branchial apparatus of this crustacean presents some very interesting approximations to the less-modified type from which it cannot be reasonably doubted that the Brachyura have proceeded. The epipodites of the three maxillipedes are very similar in form and disposition to those of the ordinary Crabs ; and that of the second bears a small horizontal branchial plume. That of the third sometimes bears a small branchia. The coxopodite of the fourth thoracic limb has a small epipodite ; but I find no trace of such an appendage on the rest of the thoracic limbs. Moreover there are five pairs of arthrobranchiae attached to consecutive somites from the ninth to |