. The elasmobranch fishes . B Fig. 129. Finer structure of a tooth, Myliohatis. (From Eose.) A. Sagittal section of whole tooth. B. Transverse section cutting a few canals; highly magnified. CO., central canals; d., dentine; , dentinal canals; e., enamel layer; vd., vasodentine. these forms the teeth are arranged horizontally in crescentic rows, several of the outer rows functioning at the same time; upon injury or loss the outermost teeth are replaced by others which migrate outward over the jaw to take their places. It may thus happen that the teeth in several rows may be lost in en- coun
. The elasmobranch fishes . B Fig. 129. Finer structure of a tooth, Myliohatis. (From Eose.) A. Sagittal section of whole tooth. B. Transverse section cutting a few canals; highly magnified. CO., central canals; d., dentine; , dentinal canals; e., enamel layer; vd., vasodentine. these forms the teeth are arranged horizontally in crescentic rows, several of the outer rows functioning at the same time; upon injury or loss the outermost teeth are replaced by others which migrate outward over the jaw to take their places. It may thus happen that the teeth in several rows may be lost in en- counter or in capture of food. In such an event, provision has been made for their replacement, for at the base of the innermost row are other deeply buried and less mature teeth ranging in development down to flabby tooth-buds. Pharynx The pharynx is that part of the tract which leads from the buccal cavity and which is characterized by branchial perforations or clefts opening through its walls. In addition to the spiracle seven such apertures perforate the pharynx in Heptanchus, six in Hexanchus and Chlamydoselachus, and five in pentanchid Elasmobranchs. In the adult the spiracle opens into the hyoidean pocket and is of small size. The remaining perforations are large but de-
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