78 The Vertebrate Organ Systems 



reception of these sensations there are special nerve endings scattered 

 through the deeper layers of the skin. Many of these nerve end- 

 ings are enclosed within special capsules. Each ending receives but 

 one type of sensation. Those sensations distinguished by the receptors 

 of the integument are touch, pain, heat, cold, and pressure. (Fig. 66.) 

 In fish and larval amphibians there is a special series of sense 

 organs located on the body and head region. Often these are arranged 

 in a single line and are known as the lateral line system. Essentially 

 these consist of pits with special sensory cells at their bases. These 

 cells apparently aid the fish in locomotion by perceiving sensations 

 related to currents and water movements. 



6. Locomotion. Among some vertebrates special structures de- 

 rived from the integument are an important aid in locomotion. The 

 wing and tail feathers of birds are skin derivatives making possible the 

 flight of this important group of animals. 



Among some other vertebrates the skin forms webs or special folds 

 to aid in locomotion. Among these are the lateral folds of skin utilized 

 by the flying squirrels, the flying lemurs, and the flying dragon of India. 

 These animals actually glide through the air utilizing these folds as 

 parachutes. The bat has its forelimb highly modified for flight. Webs 

 of skin extend between the elongated digits forming an effective flight 

 wing. Aquatic birds and frogs have webs between the toes which 

 enable them to swim rapidly. 



The fins of fish may also be considered as integumentary derivatives. 

 Here the skin is extended into a broad flap covering the small sup- 

 porting fin rays. 



7. Excretion. . To a very limited extent the skin aids the kidneys 

 in excreting waste materials. Human sweat is about 99 per cent 

 water, but in addition has salts (mainly sodium chloride) and very 

 small amounts of urea. In extremely hot weather, enough salt may 

 be lost through sweating to make it necessary to take in additional 

 quantities in order to maintain a proper balance in the body. 



Among many amphibians, the skin is important as a respiratory 

 organ, and hence essential as a source of carbon-dioxide elimination. 



8. Secretion. Located throughout the skin of mammals are nu- 

 merous glands which secrete an oily substance, sebum. This material 

 contains proteins, salts, water, and fats. This sebum is of utmost im- 

 portance in keeping the skin moist and supple, and in preventing the 

 entrance of water. It also helps to keep the hair from drying and 



