146 ORGANS AND ORGAN SYSTEMS 



receive stimuli when the worm is irritated. These are much more 

 numerous in the anterior part of the worm than elsewhere making 

 this the most sensitive or irritable portion of the whole body. 

 The irritation received by these sensory cells is passed as a 

 nervous impulse through prolongations of the sensory cells in 

 the form of nerve fibers to the central nc-^ous system which 

 runs from end to end. of the worm^ There are no aggregates of 

 sensory cells to f9rm sense organs, but the entire skin is sensitive. 



The earthworm marks a great advance in the organization of 

 the nervous system. In Hydra and the coelenterates there is 

 nothing like the highly coordinated 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, (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). 



