AND MEANS OF LIVING
ganized mass of pulp, as believed by those people whose education in such naatters comes principally from under- foot. The physical unity of all fornas of lire makes it neces- sary that every creature naust perforna the sanae vital functions. The insects have, in naany respects, adopted their own wavs of accomplishing these functions, but, as already pointC out, the naeans 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 conaplex aninaal naay 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 naay accornplish its purpose, the activities of each worker naust be coordinated with those of all the other workers by orders fro?n 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 aninaal organization is the central nervous systena. The work of almost every cell in the body is ordered and con- trolled by a "nerve inapulse" sent toit over a nerve fiber frona a nerve center. The inner structure of the nervous tissues and the work- ing mechanisna of the nerve centers are essentially alike in all animals, but the form and arrangenaent of the nerve tissue masses and the distribution of the nerve fibers naay differ nhuch 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 round it just as well for their purposes to l?ave the principal nerve cord lying free in the lower part of the body (Fig. 67, I'.VC). In the head there is a brain (Figs. ?7, 7 -?, 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 (SoeG?lg). Frona this nerve naass another pair of nerve cords goes to a third nerve mass
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INSECTS
