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Show 1904.] OF THE SPOONBILL STURGEON. 33 could be made out with regard to its nature. The specimens had been in spirit for a long time, which greatly increased the difficulty of identification. I was able, however, to recognise among it remains of parts of the exoskeleton of insect larva? together with portions of the spiral thickenings of the trachea?, a few small Oligochsete worms, fragments of plant-remains, and some earthy matter. There is every reason to believe that the gill-rakers of Polyodon are similar in function to those of Cetorhinus. On account of their fineness, and the closeness with which they are packed together, they would form an even more effective straining-apparatus than they do in the case of the latter fish. In the absence of direct observations on the living fish, as to the precise method by which the gill-rakers constitute an efficient filtering-mechanism, recourse must be had to anatomical evidence. If the gill-rakers act as a filter, it is clear that the anterior series of rakers of one branchial arch must be inclined forwards so as to meet the posterior series of the preceding arch, which have become inclined backwards for the purpose, so that the two series of gill-rakers interlock or interdigitate across the cleft. Under ordinary circumstances, however, the gill-rakers are closely applied to the anterior or posterior surfaces of the flattened branchial arches to which they belong, and they do not in the least incline across a cleft, or tend to meet those of an adjacent arch on the opposite side of the cleft. In all probability the necessary movements of the gill-rakers are brought about by means of the contractions of the muscle-fibres, represented in PI. II. fig. 5, which extend downwards and inwards from the outside of a, gill-raker and are attached to the cartilage of the branchial arch. By the contraction of these muscle-fibres the gill-rakers would be pulled outwards so as to form an angle of about 60° with the septum. When the fibres are relaxed, the gill-rakers, on account of the pull that is exerted upon them by the stretched elastic fibres, would spring back of their own accord to close against the septum, and so take up the position they occupy when not in use. The anterior row carried by the first gill-arch is composed, as already mentioned, of the largest individual gill-rakers. The reason for this appears to be that they have to bridge over the interval between that arch and the hyoid, since the latter is devoid of gill-rakers. In concluding these few notes it may be worth while to mention that the gill-rakers of the other surviving Ohondrostean Ganoids differ very greatly from those of Polyodon, although all these Fishes share the common character of having a gr eatly reduced dentition. In Psephurus they are comparatively short and are moderate in number. In Scaphirhynchus they are small fanshaped structures each of which terminates in three or four points. In Acipenser the gill-rakers are small, flattened, and pointed organs which differ somewhat in relative size among the various species. P roc. Z ool. Soc.-1904, V ol. II. No. III. 3 |
