EVOLUTION OF THE NERVOUS SYSTEM 



293 



nerve stimulus started by a small change in the environment ; a general 

 nerve network propagating the excitatory changes in all directions, but with 

 special ease in certain directions ; and a reactive muscular tissue which 

 carries out movements at the end of the chain of excitation, all the elements 

 forming the chain being derived from epiblastic cells. 



A further differentiation of a nervous system, such as that just described, 

 must in the first place involve the laying down 

 of more ' long paths ' and the collection of the 

 special ' central ' cells into closely connected 

 masses (ganglia), so as to concentrate the control 

 of the reactions of the body, and to permit of the 

 ready subordination of every part to the needs of 

 the whole. A special direction is given to this 

 development by the evolution of animals, such as 

 worms and crustaceans, which are segmented and 

 capable of locomotion. The fact that these animals 

 are segmented determines the collection of the 

 central cells into a chain of ganglia, one ganglion 

 or pair of ganglia being provided for each segment. 

 In the act of locomotion it is of advantage to the 

 animal that those sense-organs or sensory cells 

 which are proficient, i.e. which are stimulated by 

 changes in the environment originating at a dis- 

 tance from the animal, should be collected together 

 near that part which goes first, namely, the head 

 end. Thus the projected sensations of sight, 

 those which are excited by chemical changes in 

 the surrounding medium and represent the sense 

 of smell, and those which are specially aroused 

 by vibrations in the surrounding medium and 

 correspond to those which we call the sense of 

 sound, are in the majority of these animals sub- 

 served by organs situated near the head end. 



The wisdom of a man is measured by his fore- 

 sight. The chances of an animal in the struggle 

 for existence are determined by the degree to which 

 the reactions of the animal to its immediate en- 

 vironment are held in check in response to stimuli 

 arising from approaching events. An animal with- 

 out power to see, smell, or hear its enemy will re- 

 ceive no impulse to flee until it is already within 

 its enemy's jaws. It must therefore be of ad- 

 vantage to a segmented animal that the activities of the whole chain of 

 segmented ganglia should be subservient to those central nerve-cells which 

 are in direct connection with the projicient sense-organs at the head. The 

 influence exerted by the head ganglia will be in the first place inhibitory of 



FIG. 137. View of central 

 nervous system of cray- 

 fish. (After YUNG and 

 VOGT.) 



a, cerebral ganglion. 



b, commissure. 

 e,subcesophageal ganglion. 

 g. first abdominal ganglion 

 7, oesophagus. 



m, optic nerve. 



p, antennary nerve. 



,s, stomato-gastric nerve. 



