532 STRUCTURE OF VERTEBRATA 



Skin. — This forms a continuous covering over the sur- 

 face of the body, serves as a protection to the underlying 

 tissues, in some instances retains its primitive respiratory 

 significance, and is frequently concerned in the excretion of 

 waste and the regulation of the body temperature. As one 

 or other of its many functions predominates, there are cor- 

 responding structural modifications. One function which 

 we find oftenest emphasised, at the expense of the others, 

 is that of protection, and yet the extinct Glyptodon, the 

 sluggish Chelonia, the decadent " Ganoids," seem to 

 indicate that this, in itself, or in its correlated variations, is 

 not conducive to the continuance of the species. 



The skin includes — 



(a) The epidermis, usually in several layers, the outer " horny " 



stratum corneum, the inner actively growing stratum Malpighii, 



or mucosum ; both derived from the ectoderm of the embryo. 



{b) The dermis, cutis, corium, or under-skin, derived from the 



mesoderm of the embryo. 



From the epidermis are derived feathers, hairs, and some kinds of 



scales. The dermis, as is natural when we consider its origin from 



the mesoblast (mesenchyme) or vascular layer, assists in nourishing 



these epidermic structures. In the case of feathers and the scales 



of Reptiles, the dermic papilla is of primary importance, but in the 



case of hairs it arises late and is alwa5's small. From the dermis are 



derived the bony shields of armadillos and a few related mammals, 



the bony scutes of crocodiles and some other reptiles, and the scales 



of most bony fishes. This again is readily explained by the fact that 



the mesoblast is also the skeletal layer of the embryo. The ordinary 



teeth of Vertebrates, as well as the superficial or skin-teeth of gristly 



fishes, are largely formed from the dermis, but are usually covered by a 



thin coating of ectodermic enamel. 



Skeletal system. — Apart from the exoskeleton of skin- 

 teeth, scutes, shields, etc., the skeleton consists of the 

 following parts : — 



(The skull and its associated *' arches." 

 The backbone and associated ribs. 

 (The notochord is transitory except 

 in the simplest Vertebrates.) 

 (b) Appendicular/ Fore limbs, and pectoral girdle. 



Skeleton. tHind limbs, and pelvic girdle. 

 Skull. — The notochord grows forward anteriorly as far 

 as that region of the brain known as the optic thalami. 

 Around notochord and brain the mesenchyme forms a 

 continuous sheath, which is the foundation of the skull. 



