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 COLUMN. 



