WAYS AND MEANS OF LIVING 



ganized mass of pulp, as believed by those people whose 

 education in such matters comes principally from under- 

 foot. The physical unity of all forms of life makes it neces- 

 sary that every creature must perform the same vital 

 functions. The insects have, in many respects, adopted 

 their own wavs of accomplishing these functions, but, as 

 already pointed out, the means of doing a thing does not 

 count with nature so long as the end results are attained. 

 The essential conditions are the supply of necessities and 

 the removal of wastes. 



The body of a complex animal may be likened to a great 

 factory, in which the individual workers are represented by 

 the cells, and groups of workers by the organs. That the 

 factory may accomplish its purpose, the activities of each 

 worker must be coordinated with those of all the other 

 workers by orders from a directing office. Just so, the ac- 

 tivities of the cells and organs of the animal must be con- 

 trolled and coordinated; and the directing office of the 

 animal organization is the central nervous system. The 

 work of almost every cell in the body is ordered and con- 

 trolled by a "nerve impulse" sent to it over a nerve fiber 

 from a nerve center. 



The inner structure of the nervous tissues and the work- 

 ing mechanism of the nerve centers are essentially alike in 

 all animals, but the form and arrangement of the nerve 

 tissue masses and the distribution of the nerve fibers may 

 differ much according to the plan of the general body or- 

 ganization. The insects, instead of following the verte- 

 brate plan of having the central nerve cord along the back 

 inclosed in a bony sheath, have found it just as well for 

 their purposes to have the principal nerve cord lying free in 

 the lower part of the bodv (Fig- 67, VNC). In the head 

 there is a brain (Figs. 67, 72, Br) situated above the 

 oesophagus (Fig. 67, Oe), but it is connected by a pair of 

 cords with another nerve mass below the pharynx in the 

 lower part of the head (SoeGng). From this nerve mass 

 another pair of nerve cords goes to a third nerve mass 



[M7l 



