224 



COLLEGE ZOOLOGY 



ganglion, on the dorsal surface of the pharynx in segment III 

 (Fig. 156, 2). This is connected by two circumpharyngeal 

 connectives (3) with a pair of subpharyngeal ganglia which lie 

 just beneath the pharynx (4). From the latter the ventral nerve- 

 cord (Fig. 154, n.c) extends posteriorly near the ventral body- 

 wall (Fig. 153, n.co). The ventral nerve-cord enlarges into a 

 ganglion in each segment and gives off three pairs of nerves in 



every segment pos- 

 terior to IV. Each 

 ganglion really 

 consists of two 

 ganglia fused to- 

 gether. Near the 

 dorsal surface of 

 every ganglionic 

 mass are three 

 longitudinal cords, 

 the neurochords or 

 " giant fibers " 

 (Fig. 157, vg). 

 The brain and 

 nerve-cord con- 

 stitute the central 

 nervous system; the nerves which pass from and to them repre- 

 sent the peripheral nervous system. 



The nerves of the peripheral nervous system are either efferent 

 or afferent. Efferent nerve-fibers (Fig. 157, mf.) are extensions 

 from cells in the ganglia of the central nervous system. They 

 pas? out to the muscles or other organs, and, since impulses 

 sent along them give rise to movements, the cells of which they 

 are a part are said to be motor nerve-cells (mc). The afferent 

 fibers (sf.) originate from nerve-cells in the epidermis (sc) which 

 are sensory in function, and extend into the ventral nerve-cord. 

 The functions of nervous tissue are perception, conduction, 

 and stimulation. These are usually performed by nerve-cells, 



Fig. 156. — Diagram of the anterior end of an earth- 

 worm to show the arrangement of the nervous system. 

 /, prostomium ; 2, brain ; 3, circumpharyngeal connec- 

 tive; d, subpharyngeal ganglion; 5, mouth; 6, pharynx; 

 7, setae ; 8, tactile nerves to prostomium ; 9, dorsal 

 nerves; 10, ventral nerves. (From Shipley and Mac- 

 BrideJ 



