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 necessary that every creature must perform the same vital functions. The insects have, in many respects, adopted their own ways 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 activities of the cells and organs of the animal must be controlled 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 controlled by a "nerve impulse" sent to it over a nerve fiber from a nerve center.
The inner structure of the nervous tissues and the working 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 organization. The insects, instead of following the vertebrate 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 body (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 (SoeGug). From this nerve mass another pair of nerve cords goes to a third nerve mass
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