GENERAL ANIMAL FUNCTIONS. 89 



dividing into two animals is a rather common one. We 

 see it in Hydra (Fig. 46, A) and even in the worms (Fig. 

 58, 1 ). This is an expensive method of reproducing 

 and makes a great drain on the parent animal. 



In most of the higher animals the function of repro- 

 ducing new individuals is localized in certain organs 

 reproductive organs. In such cases the essential tissues 

 that are concerned are the ovary in the female (which 

 produces the ova) and the spermary in the male (which 

 produces the sperm). These cells (egg and sperm) 

 actually start the new individual (Fig. 8, 3 and 4). These 

 are the constant elements in the reproductive system; 

 the other structures, which serve to bring these two kinds 

 of cells together and to protect them during their develop- 

 ment, vary widely in the different animals. (See also 

 the next chapter.) 



95. Motion and the Muscular System. The most impor- 

 tant tasks which animals must perform are securing food 

 and escaping their enemies. Both imply motion. The 

 power of motion we have seen to reside in protoplasm. 

 The motions of all animals are derived from the power of 

 protoplasm. In one-celled animals certain parts of the 

 protoplasm often become especially contractile. They 

 may extend outside the body of the cell, as vibratile 

 cilia (Fig. 7, 6-10), or be within, as in the stalk of Vorti- 

 cella (Fig. 39, m). 



In the higher animals we find cells of both kinds. 

 Ciliated cells are found, for example, in the lining of the 

 trachea where the vibrating cilia help to free the surface 

 of foreign particles (Fig. 8, 5). In muscle cells, on the 

 contrary, the contractile elements are entirely within 

 (Fig. 8, /j). 



