28 BIOLOGY 



An alimentary canal is of little use unless it be put 

 into a position to obtain material to work upon. In 

 plants the food- materials are everywhere present, water, 

 inorganic salts, and gases from the air, but the food of 

 animals can be obtained only as the results of the work 

 of previous living organisms. It is not to be found, 

 therefore, in every place, and so the food must be 

 brought to the animal or the animal must be supplied 

 with organs to take it to the food, that is, organs of 

 locomotion must be developed. 



The uni-cellular forms of organisms seem to possess 

 a generalised locomotory power which manifests itself 

 in the pushing out of processes called pseudopodia and 

 the movement of the cell after these. The next step 

 is the development of hair-like or whip-like outgrowths 

 of the outer layer, which set up currents that carry the 

 food to the body in sedentary forms such as Vorticella, 

 or, in the case of free organisms, such as Paramoecium, 

 propel them through the medium in which they live, 

 and so enable them to obtain the necessary supply of 

 food. 



This specialisation continues, and in Ccelenterates we 

 find that cells are set aside which by their power of 

 contraction are capable of causing the animal to move 

 from place to place. There is a rapid development of 

 these contractile or muscle cells, and the final result is 

 the complex series of muscles found in higher animals. 



True organs of locomotion in connection with these 

 muscles make their appearance first in the bristles or 

 setae found in the segments of the earthworm. From 

 these simple bristles we advance to such forms as the 

 jointed limbs of insects and crabs, all the joints of which 



