. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 146 ELEMENTS OF BIOLOGY relations o£ the layers o£ the human skin and the position of the hair and glands are best understood by reference to the accompany- ing figure (Fig. 94). The Endoskeleton. The exoskeleton in the members of the phylum Chordata serves largely for protection. Support is provided by an internal framework known as the endoskeleton, which is composed of shafts and plates of cartilage or bone, tissues that are EPIDEffMISi HAIR CORNEUM MALPIGHIAN LAYER. NERVE OIL GLAND BLOOD
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 146 ELEMENTS OF BIOLOGY relations o£ the layers o£ the human skin and the position of the hair and glands are best understood by reference to the accompany- ing figure (Fig. 94). The Endoskeleton. The exoskeleton in the members of the phylum Chordata serves largely for protection. Support is provided by an internal framework known as the endoskeleton, which is composed of shafts and plates of cartilage or bone, tissues that are EPIDEffMISi HAIR CORNEUM MALPIGHIAN LAYER. NERVE OIL GLAND BLOOD VESSEL>DERMIS SWEAT GLAND FAT J Fig. 94.—Diagrammatic section through the human skin. Note that the hair, sebaceous gland, and sudiferous gland are deeply embedded in the dermis and associated with the blood supply, but that they are derivatives of the epidermis. peculiar to this phylum. Endoskeletons are unusual in achordates and never contain bone. The chordate endoskeleton always includes, some time during the life history of the animal, an axial rod of tissue known as the notochord; hence the phylum name. It is situ- ated in the body wall just dorsal to the coelom and immediately ventral to the main nerve cord, or spinal cord (Fig. 95). In mem- bers of the sub-phylum Vertebrata the notochord appears in the embryo, but is more or less completely replaced by an axis consist- ing of a series of spool-shaped cartilages or bones, known as the VERTEBRAL 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 Buchanan, James William, 1888-. New York, London, Harper & brothers
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