L6 



Basic Structure of Vertebrates 



Fig. 8. Narwhal skull. Bone has been removed to expose the root of the tusk 

 and the rudimentary tusk of the other side. (Courtesy, Flower and Lydekker: 

 ''Introduction to the Study of Mammals," London, A. & C. Black, Ltd.) 



Fig. 9. Flounder, Paralichthys oblongus (Mitchill). (Courtesy, Jordan and 

 Evermann: " Fishes of North and Middle America," Washington, The Smithsonian 

 Institution.) 



the development of the adult there occur numerous and various devia- 

 tions from this plan, either where symmetry is not necessary for func- 

 tion, or where departure from symmetry favors function, as in the re- 

 productive organs of female birds. 



Head, Neck, Trunk, Tail 



In the external form of a vertebrate, these four regions are more or 

 less definitely demarked. A fish has no obvious neck. But the gills and 

 certain glands (thyroid and thymus), situated just at the rear of the 

 head, mark a region corresponding to the anterior region of the neck 

 of a mammal. In contrast to fishes are the swan, ostrich, and giraffe. 

 The elongated flexible muscular neck of birds and mammals gives the 

 advantage of free mobility of the head. A fish, to look behind, must 

 turn the whole body. 



The trunk is hollow, and its chief cavity (body-cavity or coelom) 

 contains the various bulky viscera of the animal (Fig. 10). The coelom 

 does not extend into the neck or tail of the adult animal. 



The tail is the solid muscular extension of the animal behind the 

 hollow trunk. In most fishes the trunk tapers gradually into the nar- 



