Cassell's Natural History

Cassell's Natural History

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1913 edition. Excerpt: ...arrange themselves in concentric lines. It must not be supposed that the scales are on the surface of the skin. The skin of a fish, like that of other vertebrates, consists of two layers: an outer, cellular layer, without blood-vessels, which is called the epidermis, and a thicker internal layer, fibrous and supplied with blood-vessels, which is called the derma. The bases of the scales are situated in this inner tissue, the derma, but their posterior edges may project more or less through the epidermis. The skin of the dog-fish will offer the student possessing a microscope, and a razor wherewith to cut sections, an interesting introduction to the dermal tissues of fishes. The entire surface is covered with minute placoid scales, or dermal denticles (teeth), as they are called, each consisting of a flat base possessing the structure of bone and a pointed projecting spine. The external layer of the spine consists of enamel, below which is a layer of dentine or ivory, the characteristic substance of all teeth, and within this layer is a cavity containing vascular pulp and opening by an aperture in the basal plate; in fact, the whole scale has the same essential structure as a tooth. The late Professor Agassiz, impressed with the differences of form in the scales, at one time believed that fishes might be classified by means of them, and he proposed to divide the scales into four types: those which were bony, and formed of a thick, osseous layer, covered with hard, transparent enamel, were termed Ganoids; those dermal spines or tubercles seen in the thornback ray and many other cartilaginous fishes, which have a spine arising from a more or less circular bony base, were called Placoid scales; the scales which have the free margin more or more

Product details

  • Paperback | 168 pages
  • 189 x 246 x 9mm | 313g
  • United States
  • English
  • black & white illustrations
  • 1236504208
  • 9781236504203