208 GENERAL ZOOLOGY 



simple, very small, bilobed ganglion, joined by connectives 

 to a suboesophageal ganglion (Fig. 100, 28) ; from the latter a 

 pair of connectives extend along the ventral wall of the body- 

 cavity to the posterior end, with a ganglion (Fig. 100, 29) at 

 every somite. The brain and ventral nerve-chain constitute 

 the central nervous system. From each ganglion of the ven- 

 tral chain two pairs of nerves run out to the muscles of the 

 body-wall and the internal organs, and one pair begins in 

 the connective near by. The entire set of nerves leaving the 

 central nervous system constitutes the peripheral (surface) 

 nervous system. 



If we were to trace one of the nerves outward from the cen- 

 tral nervous system as it penetrates the muscular tissue, we 

 should find that it divides into many very fine fibers. Certain 

 fibers can be traced to the end as they merge their substance 

 with a muscle-fiber. The nerve-fiber that is united to a 

 muscle-fiber (Fig. 102, 9) is a slender portion of a nerve-cell 

 (Fig. 102, 8), the nucleus (Fig. 102, 7) of which is in the 

 central nervous system (Fig. 102, 6). Nerve-cells of this t}^pe 

 are called motor nerve-cells. The fibers of many motor nerve- 

 cells lie parallel in every nerve. ' Mingled with them is 

 another type of nerve-cell which has its nucleus (Fig. 102,2) 

 at or near the skin of the animal, and the terminal portion of 

 its principal fiber (Fig. 102, 3, 4, 5) extending into the central 

 nervous system. Cells of this type are called sensory nerve-cells. 



Reflex Action. In case an object touches one of the fine 

 sensory hairs on the skin (Fig. 102, l) of an earthworm, a 

 nervous impulse is sent along the sensory fiber to the ventral 

 nerve-chain. There the impulse is transferred to the short 

 branches of motor fibers, and by them ultimately to the prin- 

 cipal motor fiber (Fig. 102, 8). When the impulse reaches 

 the end of the nerve-fiber, the muscle-fiber (Fig. 102, 9) to 

 which it is attached contracts. When many sensory nerve- 

 cells are stimulated in this way, many motor cells will carry 



