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Show 1904.] OF THE SPOONBILL STURGEON. 23 occur as a single or double row of protuberances disposed along the concave inner or pharyngeal margins of the branchial arches. They may be modified, however, so as to become tooth-like, seti-form, or even filamentous. In some fishes they are completely absent. Among the Elasmobranchs these structures are found in a number of species, and, as a typical example, they are well seen in Acanthias vulgaris. In this fish they occur as lanceolate projections which are developed principally along the anterior edges of the pharyngeal margins of the arches. In the " Basking Shark," Celorhinus (Selache) maximus, they are found as a series of greatly elongated, coarse, seta-like structures which are disposed along the inner margins of the branchial arches upon both the anterior and posterior edges. A somewhat similar apparatus is described as being present in the South African " Whale Shark," Rhinodon typicus. Among the Holocephali the gill-rakers are small in size and are not conspicuous ; they are seen in Chimcera as rows of small tubercles. By far the greater number of the Teleostomes possess gill-rakers, and it is in this subclass that they exhibit their widest range of variation. A contribution towards a systematic study of them in the freshwater members of the Teleostomi has been made quite recently by Zander*. He has examined a considerable number of species, and finds that in carnivorous types like Esox and Lucioperca the gill-rakers are in the form of teeth. In numerous other genera they form a sievelike filtering-apparatus (" Siebfortsatze " ), which is developed in some cases upon both edges of the branchial arches, as in Perea, Acerina, Lota, and the Oyprinidse; or in others only upon the anterior edges, as in Clupea alosa, Coregonus fera , C. albula, and Osmerus. The relative fineness of the " Siebfortsatze" is correlated with the nature of the food of the species where they occur, and attains its extreme development in this respect amongst those fishes which subsist upon plankton. Popta f also has studied the gill-rakers in the Teleostomi, and has given brief descriptions of their structure and disposition in numerous species. As the result of his studies, he finds that they are specifically peculiar or diagnostic in all the forms which he examined, and interprets their arrangement and development in relation to the form of the mouth and the nature of the food. All three genera of the Dipnoi possess gill-rakers. They are largest and stoutest in Neoceradotus. In Protopterus and Lepidosiren they are present in the condition of minute, slender, pointed projections, and in the specimens which I examined they are slightly more delicate in the first mentioned of the last two genera. The object of the present article is to call attention to some interesting features with regard to the structure and function of gill-rakers in the case of the " Spoonbill " or " Paddle Fish," Polyodon spathula. I am indebted to Prof. T. W. Bridge, F.R.S., for suggesting to me that I should examine these organs, and * Zeitsch. fur wiss. Zool. Bd. lxxv. 1903, pp. 233-258. f Ann. Sci. Nat., Zool. t. xii. 1900, pp. 139-216. |
