18/8.] AND DISTRIBUTION OF THE CRAYFISHES. 7C1 



plate, which broadens at its upper extremity, and sends a short 

 process downwards beyond its articulation with the coxopodite of 

 the maxillipede. The plate is slightly folded upon itself longitu- 

 dinally, but in such a manner that it is concave forwards instead of 

 backwards. It bears no branchial papillae, and has no longitudinal 

 plaits ; but, on its posterior face and along its inner edge, it presents 

 hooked tubercles, like those of the laminae of the podobranchite. 

 It is obvious that this structure, which lies immediately behind and 

 parallel with the scaphognatbite of the second maxilla (but, as I have 

 ascertained, does not share its function of scooping the water out of 

 the branchial cavity 1 ), is a modified podobranchia, reduced, as it 

 were, to the part which, in the other podobranchiBe, is represented 

 by the base, stem, and lamina. 



Thus every thoracic limb, except the last, is provided with the 

 representative of a podobranchia — though, in the case of the first 

 maxillipede, this structure, if it plays any part in the respiratory 

 process, does so simply in virtue of its thin and soft texture, and not 

 by means of any special branchial filaments. The podobranchia of 

 the first thoracic appendage is, in fact, reduced to a mere epipodite. 



When the podobranchise are removed, six other gills come into 

 view. They are attached (fig. 3, arb) to the flexible membrane which 

 uuites the coxopodites of all the thoracic limbs to the thorax, save 

 the first and last, and may be termed anterior arthrobranchice. 

 Like the foregoing, they are disposed vertically, and increase in 

 size from the first, which belongs to the second maxillipede and is 

 hidden behind the epipodite of the first maxillipede, to the last. 

 The apex of each of these gills is exactly like the apical plume of 

 one of the podobranchiee ; and the branchial filaments are set upon 

 the outer and anterior face of the stem in the same way. The 

 inner face is flat and free from filaments ; and there is no trace of a 

 lamina or of a basal dilatation. 



Above and behind these, more directly above in the posterior, 

 more behind in the anterior limbs (fig. 3, arb), are five other branchiae 

 of similar character, attached to the arthrodial membranes of the 

 third maxillipede and the anterior four ambulatory limbs. These 

 may be termed the posterior arthrobranchice. 



After the removal of all these functional branchiae, there will be 

 found, immediately above the bases of the penultimate and antepeuul- 

 timate thoracic limbs (fig. 3, 12 plb, isplb), two minute filamentous 

 processes, the longer of which was not more than one sixth of an inch 

 in length in any specimen I have examined, while both are so deli- 

 cate as to be invisible except under a simple lens. The posterior 

 of these is the larger : it has the structure of an ordinary branchial 

 filament, wiih a somewhat swollen base, which is attached to the 

 margins of a foramen in the lower part of the epimeron of the pe- 

 nultimate thoracic somite, just below a transverse depression which 

 separates this from the upper part of the epimeron. The position 



1 If the branchiostegite of a living Crayfish is carefully removed, the rapid 

 rhythmical motion of the seaphognathite is readily seen; but the modified 

 podobranchia of the first maxillipede remains quiescent. 



