324 ZOOLOGY. 



of connective tissue, but contains in addition nerves and blood 

 vessels beside such ingrowths from the epidermis as glands, 

 hair-follicles, etc. 



339. Special Products of the Integument often occur in 

 the form of outgrowths or ingrowths. Glands are examples 

 of the latter, and are frequent in connection with the epidermis. 

 They may be simple and unicellular (mucous glands in fishes) 

 or multicellular, penetrating deep into the dermis (sweat and 

 oil glands, Fig. 153, sg). The mammary glands of Mammalia 

 are modified forms of the oil glands. The outgrowths may be 

 purely epidermal, as in hair, feathers, nails, hoofs, claws, and 

 the scales of some reptiles ; or in other instances the principal 

 structures are formed in the dermis, usually with an outer 

 layer contributed by the epidermis, as in the teeth or the 

 scales and bony plates which form in many instances (turtle, 

 armadillo, etc.) a very complete external skeleton. Some of 

 the bones of this external, or dermal, skeleton persist even in 

 the highest forms (e. g., man) and unite with bones of the 

 internal skeleton, as in the formation of the cranium and the 

 facial bones. 



The most apparent function of the skin is protection. The 

 outgrowths (hair, scales, claws, etc.) evidently increase its 

 adaptation to this function. In addition, the skin is partly 

 respiratory and excretory. The glands represent a specializa- 

 tion of this latter function. It is also sensory, and in an 

 indirect way assists in regulating bodily temperature, espe- 

 cially in the warm-blooded types. 



340. The Skeleton. Attention has already been called to 

 the exoskeleton as the derivative of the skin. The endoskele- 

 ton is surrounded by muscles separating it from the integu- 

 ment. In general it may be said that these two bony systems 

 supplement each other. In the higher forms where the internal 

 skeleton is best developed the exoskeleton is usually reduced 

 to a minimum. Elements from both sources may become fused 

 in the formation of a single structure (the skull; the carapace 



