. Outlines of zoology. Zoology. THE AXIAL SKELETON. 533 the veins by two pairs of lymph-hearts. The skin consists of a two-layered (ectodermic) epidermis, and an internal (mesodermic) dermis. The transparent outer layer of the epidermis is shed periodically, and swallowed by the frog. The dermis differs markedly from that of a fish, for there is no exoskeleton, though this was present in the extinct Labyrinthodonts; there are multicellular glands, whose secretion keeps the skin moist and is in part poisonous; and there is a stratum of unstriped muscle fibres. Pigment cells occur in the dermis,
. Outlines of zoology. Zoology. THE AXIAL SKELETON. 533 the veins by two pairs of lymph-hearts. The skin consists of a two-layered (ectodermic) epidermis, and an internal (mesodermic) dermis. The transparent outer layer of the epidermis is shed periodically, and swallowed by the frog. The dermis differs markedly from that of a fish, for there is no exoskeleton, though this was present in the extinct Labyrinthodonts; there are multicellular glands, whose secretion keeps the skin moist and is in part poisonous; and there is a stratum of unstriped muscle fibres. Pigment cells occur in the dermis, and some extend between the cells of the epidermis. The colour changes a little according to the state of these cells, the protoplasm expanding and con- tracting partly through the direct in- fluence of light and moisture on the skin, partly by a more complex reflex action in which the eyes, the brain, and the sympathetic nervous system are all implicated. In the larval sala- mander the pigment cell seems to contract and expand as a whole, but this is not usually the case. There are cutaneous blood vessels, by means of which the frog can, to a certain extent, breathe by its skin. The tadpole has sensory cells in distinct lateral lines, but of this regularity the adult retains little trace, though it has many nerve-end- ings and " touch spots" in various parts of its skin. The axial skeleton.—The vertebral column consists of nine vertebrae, and an unsegmented urostyle or coccyx. The first vertebra bears two facets for the two condyles of the skull, and an odontoid process which lies between the condyles. It has no transverse processes, and its arch is in- completely ossified. Each of the next six has an anteriorly concave or proccelous centrum, a neural arch surrounding the spinal cord, a transverse process from each side of the base of the arch, an anterior and a posterior pair of articular. Fig. 226. — Vertebral column and pelvic girdle of bull-frog. /./., Trans
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
