20 



THE NERVOUS SYSTEM 



The sensory cells are not so directly connected with muscle-fibers hi the 

 worms as in the sea-anemones, for between receptor and effector there is here 

 interposed a central nemous system. This system, as it appears in the earth- 

 worm, is illustrated in Fig. 2. It consists of a cerebral ganglion dorsal to the 

 buccal cavity and a row of ventrally placed ganglia bound together by a ventral 

 nerve cord. The most anterior of the ventral series of ganglia is connected to 

 the dorsal one by nerve strands on either side of the esophagus. The ganglia 

 of the ventral cord are placed so that one occurs in each body segment, and 

 from each three pairs of nerves run to the skin and muscles of that segment. 

 The arrangement of the constituent elements can best be studied in transverse 

 sections (Fig. 3). The sensory cells are located in the skin, and from each of 

 them a fiber runs along one of the nerves into the ganglion, within which it 

 branches, helping to form a network known as the neuropil. Within each 



Fig. 3. Transverse section of the ventral chain and surrounding structures of an earthworm: 

 cm, Circular muscles; ep, epidermis; Im, longitudinal muscles; me, motor cell-body; mf, motor 

 nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. (Parker.) 



ganglion are found large nerve-cells from which fibers run through the nerves 

 to the segmental musculature. Here we have the necessary parts for the sim- 

 plest reflex arc. Stimulation of the sensory cell causes nerve impulses to travel 

 through its fiber to the neuropil, thence to a motor cell, and finally along a proc- 

 ess of the latter to the muscle. In other words, we have a receptor, conductor, 

 center, another conductor, and finally an effector; and all this is for the purpose 

 of bringing the muscle-fiber under the influence of such environmental changes 

 as are able to stimulate the sensitive receptor. 



In addition to the primary sensory and motor elements just enumerated the 

 ganglia contain nerve-cells the fibers of which run from one ganglion to another 

 and serve to associate these in co-ordinated activity. These internuncial ele- 

 ments serve to establish functional connections among the different parts of 

 the ganglionated nerve cord that constitutes the central nervous apparatus; 



