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Show 1904.] OF THE SPOONBILL STURGEON. 25 again following the general rule, they are attached to the anterior and posterior surfaces, close to the concave inner margin, in the form of an anterior and a posterior series, but from the obliquity of the surfaces of the arch the two series appear as if disposed along the outer and inner faces of an arch. They will be referred to in future, however, as the anterior or the posterior series, as the case may be. If the gill-rakers are surveyed from the first branchial arch backwards to the fourth arch, they are seen to become progressively shorter in length, and, furthermore, those of them that are situated along the outer or anterior aspect of an arch are somewhat longer than those carried along the inner or posterior aspect of the same arch (PI. II. fig. 2, i.g.r. and o.g.r.). Hence the series of the longest gill-rakers is carried on the outer aspect of the first branchial arch, while the row borne on the inner aspect of the fourth branchial arch is composed of the shortest gill-rakers. It is also worthy of note that the longest gill-rakers in either an anterior or a posterior series of a branchial arch are those situated nearest to the junction of a ceratobranchial with an epibranchial element (fig. 1, r.), the gill-rakers gradually increasing in length from the dorsal and ventral extremities of an arch until the centre of the concavity is reached, where they attain their maximum. The fifth branchial arch carries gill-rakers along its anterior surface only, and they are slightly longer than those disposed along the posterior aspect of the preceding arch. The fifth gill-arch is itself much reduced, since it retains only its ceratobranchial element, and, in correspondence with this, gill-rakers are not developed on the opposing face of the preceding arch in relation to its epibranchial cartilage, but only with the ceratobranchial *. Owing to the extraordinary compression of the plate-like branchial archest, combined with the attachment of the gill-rakers to their concave inner margins, the greater portion of each arch practically forms a stout cartilaginous septum, which separates the anterior from the posterior series of gill-rakers in relation with each arch, much in the same way that an inter-branchial septum would sepaiate the double series of gill-filaments on the opposite or outer margin of a branchial arch (figs. 1 & 2). The necessity for this curious modification is by no means obvious. It would seem that, were the septum absent, such delicate and fragile organs as the gill-rakers would be very liable to get dislocated or clogged together, and perhaps damaged, through one series of gill-rakers rubbing against the other. As it is, each row is kept in a beautifully regular order, and not a single gill-raker will be noticed to be disarranged from its proper position, and all of them when not in use are closely applied to the surface of the septum. The function of this septum * A full account of the skeleton of the visceral arches of Polyodon is given by Prof. T. W. Bridge in the Phil. Trans. 1878,vol. 169; vide pp. 702-712 & pi. 57. tig's. 8 & f Lacepedc (1798) speaks of the branchial arches as cartilage-plates. |
