784 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 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 arthrobranchiae. 



The podobranchia of the first maxillipede is represented only by 

 the large curved epipodite, which sweeps over the surface of the 

 arthrobranchiae and the pleurobranchise. The podobranchia of the 

 second maxillipede is divided into a branchial plume, which lies 

 horizontally under the bases of the four anterior arthrobranchiae, 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 arthrobranchiae. 



The Branchial formula of Cancer pagurus. 



A considerable reduction of the branchial apparatus occurs in some 

 Bracbyura, especially the Catometopa. Thus, in Gelasimus the 

 hinder pleurobrauchia and the most anterior arthrobranchia have 

 disappeared, and the two podobranchiee are so small as to be almost 

 rudimentary. 



In the Raninidse 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 Bracbyura 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 some- 

 times bears a small branchia. The coxopodite of the fourth thoracic 

 limb has a small epipodite ; but I find no trace of such au appendage 

 on the rest of the thoracic limbs. Moreover there are five pairs of 

 arthrobranchiae attached to consecutive somites from the ninth to 



