. Chordate anatomy. Chordata; Anatomy, Comparative. I30 CHORDATE ANATOMY In elasmobranchs, on the edges of the jaws, these minute scales become enlarged into formidable biting teeth or sometimes, inside the mouth, into bony pavements that are used for grinding the food. That the biting tooth of elasmobranchs is a modified placoid scale is obvious from inspection, since the two look alike, and there is a transition in size and form between them. This identity is further borne out by OLFACTORY PIT -- TASTE PAPILLA. VILLUS Fig. 121.—The pharynx of an elasmobranch (Squalus) laid open
. Chordate anatomy. Chordata; Anatomy, Comparative. I30 CHORDATE ANATOMY In elasmobranchs, on the edges of the jaws, these minute scales become enlarged into formidable biting teeth or sometimes, inside the mouth, into bony pavements that are used for grinding the food. That the biting tooth of elasmobranchs is a modified placoid scale is obvious from inspection, since the two look alike, and there is a transition in size and form between them. This identity is further borne out by OLFACTORY PIT -- TASTE PAPILLA. VILLUS Fig. 121.—The pharynx of an elasmobranch (Squalus) laid open to show the double row of teeth in both upper and lower jaws. Such teeth differ only in size from the placoid scales of the pharynx and skin. Elasmobranch teeth, like scales, are fastened in the skin and are not attached to the jaw cartilages. (After Cook.) other evidence. Like true teeth, placoid scales have a base of dentine, which contains a pulp-cavity filled with connective tissue. Both scales and teeth have a spinous process, covered with enamel, which protrudes through the skin. Moreover, their development is similar in that, in both, the enamel is secreted by the ectoderm and the dentine by mes- enchyme, and both arise in that portion of the mouth where the ectoderm has invaginated to line the digestive tube. See Fig. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son & Co.
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