62 THE BIOLOGY OF AN ANIMAL. 



general surface, which then carry on the gaseous exchange for the whole 

 organism. In many aquatic animals such regions bear filaments or flat 

 plates or feathery processes known as gills or branchiae, which are bathed 

 by the water containing dissolved air, though in many such animals 

 respiration takes place to some extent over the general surface as well. In 

 insects the respiratory surface is confined to narrow tubes (trachece) which 

 grow into the body from the surface and branch through every part, but 

 must nevertheless be regarded as an infolded part of the outer surface. 

 In man and other air-breathing vertebrates the respiratory surface is 

 mainly confined to the lungs, which are simply localized infoldings of the 

 outer surface specially adapted to effect a rapid exchange of gases between 

 the blood and the air. 



It is easy to see why special regions of the outer surface have in higher 

 animals been set aside for respiration. It is essential to rapid diffusion 

 that the respiratory surface should be covered with a thin, moist membrane, 

 and it is no less essential that many animals should be provided with a 

 firm outer covering as a protection against mechanical injury or desicca- 

 tion. Hence the outer surface becomes more less distinctly differentiated 

 into two parts, viz., a protecting part, the general integument ; and a 

 respiratory part, which is usually preserved from injury by being folded 

 into the interior as in the case of lungs or tracheaB, or by being covered 

 with folds of skin as in the gills of fishes, lobsters, etc. This covering or 

 turning in of the respiratory surfaces brings with it the need of mechanical 

 arrangements for pumping air or water into the respiratory chamber ; and 

 thus arise many complicated accessory respiratory mechanisms. 



B. ORGANS OF RELATION. (For A see p. 49.) 



Motor System. The movements of the body have a twofold 

 purpose. In the first place they enable the animal to alter its 

 relation to the environment, to move about (locomotion), to seize 

 and swallow food, and to perform various adaptive actions in 

 response to changes in the environment. In the second place, 

 the movements may alter the relation of the various parts of the 

 body one to another (visceral movements and the like), such a& 

 the movements which propel the blood, drive the food along the 

 alimentary canal and roll it about (p. 49), those which expel 

 waste matters from the nephridia, discharge the reproductive 

 products, etc. 



Most of these movements are performed by structures known 

 as muscles, which consist of elongated cells (fibres) endowed in a 

 high degree with the power of contractility i.e., of shortening, 

 or drawing together (cf. p. 27). Ordinary "muscles' are in 



