FORM OF THE BODY 1 29 



the light, and for maximum efficiency this demands a branchin<^, 

 or what may be called a diffuse form of body. Since animals 

 do not demand such light exposure the branching form of body 

 is also not necessary. As a matter of fact the animal body is 

 not only not diffuse, it is constructed in the most comj^act 

 manner possible. The reason for this is, of course, not far to 

 seek. It is demanded by the most distinctively animal char- 

 acteristic — locomotion. For the purposes of respiration animals 

 also require a large exposure of surface to the surrounding 

 medium, but this is secured in the gills and lungs by folding 

 surfaces in such a way as to make the respiratory organs occupy 

 very little space in proportion to the surface which they expose. 

 This of course secures protection to the organs but at the same 

 time it also allows greater freedom of motion. That the latter 

 is an important consideration is evidenced by the fact that many 

 fixed animals are also diffuse in form. 



309. Locomotion.- — The ability to move from place to place 

 is the most conspicuous animal character, but coordinate with 

 it and inseparably connected with it is sensibility to external 

 influences. This latter character is not wanting in plants but 

 it is so much more greatly developed in animals as to amount 

 practically to a different thing. Locomotion and sensibility 

 go hand in hand because locomotion without sensibility would 

 be aimless and sensibiHty without the power of motion would 

 be without value. In this connection ''motion" or "loco- 

 motion" must be understood in a broad sense as a muscular 

 response which may involve only a part of the body. The power 

 of locomotion carries with it a large train of interesting conse- 

 quences which determine the form and structure of the animal 

 even to the minutest detail. These will be considered at 

 various points as the subject develops, but here we will examine 

 only into the matter of the external form as resulting from 



locomotion. 



310. Axis of Locomotion. — In the more primitive condition, 



9 



