138 FOUNDATIONS OF BIOLOGY 



throat, which, however, in air-breathing Vertebrates disap- 

 pear before the adult condition is attained. The trunk forms 

 the body proper and its cavity, or coelom, contains the ali- 

 mentary canal, opening to the exterior by the anus, as well 

 as the chief circulatory, excretory, and reproductive organs. 

 The tail comprises the region posterior to the coelom and 

 anus. (Figs. 74, 75.) 



In aquatic forms thin extensions from the trunk and tail 

 form median and paired FINS, the latter comprising the 

 PECTORAL fins, situated near the junction of head and trunk, 

 and the PELVIC fins, just lateral to the anus. The pectoral 

 and pelvic fins, or the fore-limbs and hind-limbs which re- 

 place them in all forms above the Fishes, are the only lateral 

 appendages found in Vertebrates. 



2. Skin 



The surface of the body which comes in direct contact 

 with the environment is covered by an integument, or SKIN, 

 which, though primarily protective and sensory in function, 

 takes part to a greater or less degree in respiration, excretion, 

 and secretion. Scales, feathers, claws, horns, hoofs, nails, 

 teeth, etc., are derivatives of the skin. The skin, unlike that 

 of the Invertebrates, is formed of two layers; an outer EPI- 

 DERMIS derived from the ectoderm, and an inner DERMIS from 

 the mesoderm of the embryo. (Fig. 76.) 



3. Muscles 



The body wall proper is chiefly composed of MUSCULAR 

 TISSUE, commonly spoken of as 'flesh/ which varies in thick- 

 ness in different regions of the body. In the mid-dorsal re- 

 gion it surrounds the CENTRAL NERVOUS SYSTEM and the 

 axial supporting structure (NOTOCHORD), while ventrally it 

 forms the wall of the coelom. In the lower Vertebrates and 



