324 



METAZOAN PHYLA 



345. Skin. — Vertebrates are covered with a skin, or integument, 

 consisting of two parts — an outer epidermis, derived from the ectoderm 

 and consisting of stratified pavement epitheUum; and an inner dermis, 

 mesodermal in origin and consisting largely of connective tissue (Fig. 

 225). The epidermis has only a few nerve endings, which are in the 

 deeper layers, and no blood vessels. The deepest layer is made up of 

 living cells which regularly undergo division and replace the dead, 

 horny, scale-like cells which are continually being lost from the surface. 

 These deep cells are supplied with blood from a dermal capillary network 

 lying just below the epidermis. From the epidermis are derived certain 



//a/r Hair 



follicle 



Sebaceous 

 gloind 



' rierye I Sweat duct 



Epidermis 



Dermis ■' 



Tactile 

 '^•1 ' corpuscle 



Erector 

 muscle ofrne 

 hair 



Sweat 

 gland 



Blood 

 vessel 



Fat 



Blood vessel Nsrve 



Fig. 225. — Somewhat diagrammatic section of the mammalian skin. 



external skeletal structures such as horny scales, feathers, hairs, nails, 

 hoofs, and claws and the enamel of the teeth. 



The dermis, or corium, contains nerve endings, and specialized groups 

 of epidermal cells forming tactile organs. It also contains numerous 

 glands, which are epidermal in origin, such as the mucous glands of fishes 

 and amphibia and the sweat, oil, and milk glands of mammals. Bones 

 are sometimes developed in the dermis. From the dermal layer are also 

 derived the dentine and the cementum of the teeth, and bony scales of fish. 



346. Skeleton. — The skeleton of vertebrates is divided into an exo- 

 skeleton and an endoskeleton. The former includes many dermal cartilages 

 and bones, to which may be added the epidermal skeletal structures; and 

 the latter, all the deeper-lying skeletal parts. The endoskeleton is made 

 up of axial and appendicular portions (Fig. 226). The former includes 

 the skull, the vertebral column, the ribs, and in the higher forms a 



