328 



ZOOLOGY 



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, especially in the 

 warm-blooded types. 



348. The Skeleton. — Attention has already been called to 

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



FiG. 156. 



Fig. 156. Diagram of transverse section through embryo of a Vertebrate, showing the mode 

 of origin and the relations of the notochord, nervous cord, ectoderm, entoderm and mesoderm (see 

 also Pig. is), coe., coelomic pouches; ect., ectoderm; ent,, entoderm; g, lumen of the gut; ic., invagi- 

 nation of ectoderm which forms the nerve cord (see c, in succeeding figures) ; mesK, somatic or body 

 mesoderm; mes*,, splanchnic mesoderm, that portion of the mesoderm which becomes allied with 

 the entoderm; n, the nerve (spinal) cord; n.c, notochord, arising by an outpocketing of the entoderm. 



ton is surrounded by muscles separating it from the integument. 

 In general it may be said that these two bony systems supple- 

 ment 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 

 of the turtle). A difference between the internal and the 



