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Show 30 MR. A. D. IMMS ON THE GILL-RAKERS [May 3, gill-rakers with his account, I have not been able to make out any salient points of resemblance between the two structures. With the exception of the rhombic plates and " fulcra " of the tail, the scales are too degenerate to admit of a similar comparison being extended to them. The plates and " fulcra " of the tail are, however, tolerably well-developed structures. The former I have examined after having thinned them, by rubbing down on the surface of a fine hone, in the same way in which the gill-rakers were treated. The matrix of a scale is colourless in thin slices and is pervaded everywhere by lacunae which are similar to those found in a gill-raker, but it does not contain any blood-channels. The substance of the plate appears to be deposited around a longitudinal core-like centre in the matrix. Adjacent plates are united to one another by means of ligamentous connections the fibres of which penetrate deeply into their matrix. These fibres are comparable to what Hertwig calls the " Schuppen-ligament" of the scales of Lepidosteus. In structure there is, therefore, a considerable likeness between a rhombic plate and the basal portion of a gill-raker. The matrix and its lacunae are identical in both cases; the hollowed core or cavity in the base of the gill-raker might be compared to the core of one of the plates, and to this may be added the absence of blood-channels in both cases. The fibres which connect a gill-raker to the branchial arch, and also which bind adjacent ones together, are comparable with the ligaments which unite neighbouring scales. For these reasons I think that it is not improbable that the basal portion of each gill-raker is the homologue of a ganoid scale-i. e., of one of the rhombic plates which are found along the sides of the upper lobe of the tail. The shaft or principal part of a gill-raker may correspond to a greatly elongated spine, or to one of the evanescent spines which are found in relation with each scale in the developing Lepidosteus, and which are regarded as representing the spinous portions of placoid scales. In Lepidosteus, as Nickerson has pointed out, the basal plate, which is the essential part of a ganoid scale, has come to be developed independently of the vestigial spines, instead of being a continuation of the process by which the latter are produced; and, in comparison with the basal plate of the Selachian scale, it has increased greatly in size and importance and has incorporated within itself fibres from the dermis*. In a gill-raker, it would seem that we have a basal plate which is similarly specialised, though not to so great an extent, but that there has been no corresponding reduction in the spinous portion, which, on the contrary, has become greatly elongated. It has no trace, however, of a hard dentine layer, nor of a coat of enamel or ganoin, unless the covering of mucous membrane is to be looked upon as the representative of the latter, * Bull. Mas. Comp. Zool., Harvard, vol. xxiv. 1893, pp. 115-140. |
