260 THE EVOLUTION OF MAN. 



cutis, Ih). The lower layers of this tough and thick 

 covering consist principally of fat and loose connective 

 tissue ; the outer layers of skin-muscles and firmer connec- 

 tive tissues. It covers the whole surface of the fleshy body, 

 with which it is connected, and it lies immediately below the 

 thin outer skin (epidermis, oJi). In the case of the higher 

 Vertebrates, hairs, nails, feathers, claws, scales etc., arise 

 from this outer skin. With all its appendages and pro- 

 ducts, it consists entirely of simple cells, and contains no 

 blood-vessels. Its cells are connected with the ends of the 

 sensory nerves. Originally the outer skin (epidermis) is an 

 entirely simple covering for the outer surface of the body, 

 and consists of but one kind of cell. In higher Vertebrates, 

 it afterwards separates into two strata, an outer, firmer 

 horn-straturn, and an inner, softer mucous stratum ; many 

 external and internal appendages arise from it at a later 

 period ; the hair, nails, etc., externally, and the sweat and 

 sebaceous glands internally. 



In the primitive Vertebrate the skin probably arose 

 along the middle line of the body in the form of an erect, 

 perpendicular seam used for floating purposes (/). The 

 Amphioxus and the Cyclostomi yet retain a similar seam, 

 which passes almost entirely round their bodies ; one is also 

 found on the tail of the larval Frog, or Tadpole (Fig. 194). 



From these external parts of the vertebrate body we 

 will now turn to the inner organs, which we find beneath 

 the notochord, in the large body, or intestinal cavity. To 

 avoid confusion, we will in future call this cavity the 

 codom.d. In Anatomy it is usually called the pleuro-peri- 

 toneal cavity (Fig. 58, c). In Man and all other Mammals, 

 but in no other animals, this ccelom, when developed, is 



