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Show 1904.] OF TIIE SPOONBILL STURGEON. 29 contains large numbers of coarse clentine-tubes which arise from the walls of the canals and run outwards to the periphery, where they form a layer of hard dentine. Turner regards these gill-rakers as being composed of vaso-dentine, but Tomes in referring to them adds " (? osteo-dentine)." * It is worthy of note that in fishes of the genus Chcetodon and their allies the maxillary teeth appear to have been modified along the same lines as those by which the setiform type of gill-raker has been produced. As their name implies, the teeth of these fishes are bristle-like; they resemble the hairs of a fine brush in being flexible and elastic, and they are composed of a yellowish, shining, semi-transparent tissue f . I would suggest that possibly the gill-rakers of Polyodon are morphologically the much modified descendants of exoskeletal structures which have migrated along with the ectoderm on to the branchial arches. The fact that the mucous membrane covering the branchial arches is regarded as being endodermal in origin, offers considerable difficulty to any idea that such structures could have developed there independently and in situ, unless they have arisen in the underlying mesoblast. Klaatsch, however, from a study of the placoid scales in Mustelus and some other Elasmo-branchs, has arrived at the conclusion that their scleroblasts are ectodermal in origin and are derived from the same layer as that which gives rise to the enamel. This layer, which is at first homogeneous, becomes divided into a portion which has been usually considered to be of mesodermal origin, while the rest remains in connection with the ectoderm+. Hence he considers that a " dermal " exoskeleton is not mesodermal in its ultimate origin. It is worthy of note that, with regard to the pharyngeal teeth of many fishes, several writers are inclined to believe that their presence is due to a migration of the ectoderm into the cavity of the pharynx. For this reason, and on account of the difficulty of reconciling them with the presence of anything except ectoderm, I would suggest the possibility that the skeletal tissue of the gill-rakers of Polyodon has arisen from portions of the epiblast forming the outer portions of the gill-clefts, which have migrated on to the inner or pharyngeal margins of the branchial arches. At all events, if any migration of epiblast has taken place, the latter route seems at least as feasible as a backward migration from the stomodseum. In Cetorhinus the gill-rakers retain many structural features in common with the teeth of the animal, but in Polyodon they appear to have undergone a more special modification along lines of their own. The structure of the teeth in the young Polyodon has been described by Zograff§, but, after a comparison of the * ‘ Dental Anatomy,' p. 220. + Vide Owen, ‘ Odontography,' pp. 8 & 105, pi. i. fig. 2. J Morph. Jahrb. xxi. 1894, pp. 153-240. § " Ueber die Zahne der Knorpel-Ganoiden," Biol. Centralbl., Bd. vii. 1887-88, p. 181. Ann. Sci. Nat., Zool. 8 ser. t. i. p. 203, pi. 4. figs. 3, 4, & 6. |
