| OCR Text |
Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 7(i 1 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 longitudinally, 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 podobranchiae. It is obvious that this structure, which lies immediately behind and parallel with the scaphognathite of the second maxilla (but, as I have ascertained, does not share its function of scooping the water out of the branchial cavity1), is a modified podobranchia, reduced, as it were, to the part which, in the other podobranchiae, 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 podobranchiae are removed, six other gills come into view. They are attached (fig. 3, arb) to the flexible membrane which unites the coxopodites of all the thoracic limbs to the thorax, save the first and last, and may be termed anterior arthrobranchiee. 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 podobranchiae; 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 arthrobranchiee. After the removal of all these functional branchiae, there will be found, immediately above the bases of the penultimate and antepenultimate thoracic limbs (fig. 3, \iplb, 13 plb), 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 delicate as to be invisible except under a simple lens. The posterior of these is the larger: it has the structure of au ordinary branchial filament, with a somewhat swollen base, which is attached to the margins'of a foramen in the lower part of the epimeron of the penultimate 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 rhvthmical motion of the scaphognathite is readily seen; but the modified podobranchia of the first maxillipede remains quiescent, |
