NERVOUS SYSTEM 185 



addition to the ventral nerve chain, the worm has a ganglion 

 which is not duplicated and which serves as a centre for the 

 entire system. This ganglion is located in the cephalic lobe, 

 on the dorsal side of the oesophagus. It is called the brain or 

 supra-cesophageal ganglion. It is connected with the ventral 

 chain by a pair of connecting nerves, which pass around the 

 oesophagus and unite with the first ventral, or sub-cesophageal 

 ganglion. The nerves from the eyes, tentacles, cirri and other 

 special sense organs of the head region, connect with the brain. 

 The central nervous system of Worms is usually sharply marked 

 off from surrounding tissues, and is enveloped in a special 

 protective connective tissue. 



439. The nerve elements are well differentiated and fall 

 naturally into three distinct classes: i. The sensory cells 

 (see page 360), which are called receptors, always lie outside the 

 central nervous system, and send a fibre into the ganglion of the 

 corresponding segment. 2. The connecting fibres, called also 

 association fibres, lie wholly within the central nervous system 

 and serve as a connection between the cells of the first and third 

 classes. The cells of these fibres lie in the ganglia. 3. Cells of 

 the third class lie in the ganglia, but send fibres out to the mus- 

 cles, glands and other organs of response. These cells are 

 called effectors. 



440. Where the receptor fibres end in the ganglia, they 

 divide into a tuft of small branches, which end in contact with 

 similar branches of other elements or with the body of another 

 cell. The fibres of related cells are also often intimately con- 

 nected where no branches occur. In this way, physiological 

 connection is made between nerve cells and the stimulus 

 transmitted from one to another. The effectors, where they 

 end on muscles or glands, also break up into numerous small 

 branches, which terminate in small disc-like enlargements in 

 contact with the response organs. 



441. The mechanism of response is then as follows: When a 



