OCR Text |
Show 774 PROF. T. II. HUXLEY ON THE CLASSIFICATION [Julie 4, approach the characters of the branchia? of the Lobster. The groove on the inner face of the stem seems, at first, to be reduced to nothing in the upper part of the podobranchiae. However, a transverse section (fig. 7, in) shows that it exists and is bounded by two lips, the anterior of which is produced into a narrow ala. But the numerous hooked branchial filaments, which arise from the posterior face of the stem, fill the groove and appear to obliterate it. The hindermost of the arthrobranchiae is attached close to the articulation of the coxopodite with the epimeron, and is therefore in the same position as one of the anterior series of arthrobranchiae in other Crayfishes. There is not the slightest trace of the attachment of another branchia on the arthrodial membrane behind this. But on the epimeron, just above the margin of the articular cavity, and 0*1 inch behind the root of the anterior branchia, a minute filament, 0*05 in. long, which I take to be a rudiment of the posterior branchia, enclosed by the downward extension of the epimeron, projects. The attachment of the next arthrobranchia is like that of its predecessor, but not quite so far forward. At a distance of 0*5 in. behind and above its root, an exceedingly minute papilla, not more than a fifth of the length of the foregoing, is enclosed in the articular edge of the epimeron. This is doubtless the rudiment of the posterior arthrobranchia. The three remaining arthrobranchia? are fixed into the arthrodial membranes of the other ambulatory and prehensile limbs, in a position corresponding with that occupied by the fourth. In the case of the hindermost of these, or that of the eleventh pair of appendages, there is, above and behind the root of the gill, and 0*08 inch distant from it, a filamentous rudimentary branchia, rather less than 0* 1 inch long, attached within the articular margin of the epimeron. Three or four minute unequal processes are given off from the posterior edge of this filament. There is another very small papillary rudiment immediately above and behind the root of the next arthrobranchia; but none could be discovered above that of the most anterior arthrobranchia. In addition to the five conspicuous and functional outer arthrobranchiae, there was another very small one, only 0*18 inch long, and almost rudimentary, attached to the second maxillipede. The pleurobranchia of the last thoracic somite was only 0*3 inch long, and was hardly visible at first, from being overlapped by the hindermost podobranchia. I could not discover the slightest trace of any other pleurobranchiae. The upper part of the modified podobranchia of the first maxillipede is sharply folded upon itself longitudinally ; and its outer lobe is beset with numerous short branchial filaments, most of which have terminal hooks. Similar hooked filaments are to be found about the base and the posterior region of the other podobranchiae ; but a large proportion of the filaments of these branchiae, and all of those of the arthrobranchiae, appear to be hookless. The tubercles of the coxopodites give rise to small tufts of short and straight seta?, which are hooked at the end, like those of Asia- |