150 ORGANS AND ORGAN SYSTEMS 



nothing like the highly co-ordinated nervous system of the 

 annelids and higher types generally. Nerve cells are present, it 

 is true, in Hydra, and, as we have seen, they form a more or less 

 complete nervous network throughout the organism, but they 

 consist of sensory cells and isolated nerve cells whose processes 

 connect with the equally isolated epithelio-muscle cells. 



In the earthworm, in common with all higher animals, the 

 sensory cells do not connect in this direct manner with the mus- 

 cles, but act through a central nervous system. The condition 

 in Hydra may be compared with a primitive telephone system, 

 where everyone rushes to the phone whenever a bell rings in the 

 system, while the earthworm condition may be compared with a 

 well-equipped and efficient modern telephone plant, where all 

 peripheral calls are sent directly to the central exchange and 

 there properly classified and transmitted. We distinguish, 

 therefore, two distinct parts of the nervous system of the earth- 

 worm (i) the sensory or peripheral system, described above, 

 and (2) the central nervous system. 



The Central Nervous System. The central nervous system 

 consists of a double nerve cord lying below the digestive 

 tract, with a large double swelling in each somite. These 

 swellings are termed ganglia, and they are made up of masses 

 of nerve cells. The ganglia are connected from somite to somite 

 by the heavy double nerves termed commissures, so that the 

 entire worm is bound together by a continuous system of com- 

 missures and ganglia which form a fairly homogeneous central 

 nervous system (Fig. 60). 



The central nervous system is connected with the sensory 

 cells of the body wall, and with all of the organs of the somites, 

 by three pairs of nerves which leave the ganglia as shown in 

 Fig. 60. These nerves are supported by the dissepiments and 

 body wall and branch and sub-branch, until they are lost in a 

 network of fine fibers penetrating muscles and epithelium. 

 Each of the main nerves is a bundle of fine fibers which transmit 

 sensory impulses to the ganglia, and motor impulses from the 

 ganglia. 



At the anterior end of the ventral nerve chain there is a pair 



