288 



LEISURE-TIME STUDIES. 



medusae, again, the lines of discharge may be traced as 

 having become definite nerve-tracts; actual nerve-elements 

 having been demonstrated to occur in Aurelia. When these 

 higher jelly-fishes, such as the Sarsice, one of which is 



represented in Fig. 52, are 

 stimulated, their actions are seen 

 to be still more purposive and 

 direct, and more quickly mani- 

 fested, than in forms in which 

 the nerve-impulses travel along 

 less definite pathways. And in 

 conformity with the higher struc- 

 ture of their nervous system, 

 the task of destroying their irri- 

 tability is easier than that of 

 annihilating the sensitiveness of 

 their lower neighbours. 



Have we, then, elucidated, 

 through the consideration of the 

 history of the jelly-fishes, any 

 points which will assist us in 

 framing a reasonable conception 

 of the origin of nerves ? The 

 amoeba, let us remember, repre- 

 sents a mass of sensitive pro- 

 toplasm, through which impulses 

 pass in an indefinite manner, 

 with the result of producing 

 irregular contractions of the 

 animalcule's body. The nerve- 

 power has its beginning here, 

 but nothing more; and any or 

 every part of the body may in 

 turn be brought into contact 



with the outer world. With a less changeable and more 

 definite shape of body, some parts of an animal of necessity 

 become more exposed than other parts to the outer world 



FIG. 52. Sarsia tnbulosa, showing 

 the central polypite and the 

 tentacles. 



