NERVOUS SYSTEM. 73 



are not entirely passive agents, but are endowed with a pecu- 

 liar property (neurility). 



The simplest expression of a nervous system is a ganglion 

 placed in relation with a pair of nerves. One of these, ex- 

 tending from the tissues (perceptive areas) to the ganglion, is 

 designed to convey an impression to the centre generating 

 power, and hence is called afferent, or sensory. The other, 

 extending from the ganglion to the tissues (excito-motor 

 areas) is supposed to transmit the power excited to activity 

 by the reception of sensation, and hence is called efferent, 

 or motor. To the transfer of an impression either way the 

 continuity of the conductor is essential. 



The complexity of a nervous system is determined by the 

 number, arrangement, and degrees of development of its 

 ganglia and commissures. 



PROTOZOA. No nervous system apparent.* 



RADIATA. Ccelenterata. Form of nervous system not 

 determined. 



Echinoclermata. The plan is that of a number of ganglia 

 arranged in the form of the circle or pentagon, connected by 

 commissures and sending branches to the surrounding tissues. 

 In Asteracanthion (star-fish) the ganglia are situated at points 

 between the rays. 



MOLLUSCA. A number of ganglia generally surround the 

 gullet, and constitute the ' O3sophageal ring.' Other ganglia 

 are seen at various points of the body connected with the 

 above, and with one another by commissural threads, their 

 position and size being determined by the relations and im- 

 portance of the regions concerned. Processes of the cell wall 

 often extend far into the nerves, while the distinct neuri- 

 lemma contains orange or reddish pigment cells. 



Polyzoa. No nervous system has with certainty been 

 recognized. 



Brachiopoda. An cesophageal ring, composed of two im- 



* Animals having ro nervous system are by some included in a group 

 named the Acrita. 



