EXPERIMENTS ON WORMS 



77 



__J Tl. 



dinal nerves contain a greater number of ganglion- 

 cells, so that they almost form segmental aggregations. 

 From this similarity we should infer that the brain- 

 functions of the fresh-water Plana- 

 rians would be analogous to those i^M' - 

 of the Polyclads. However, such 

 is not the case. If we divide a 

 fresh-water Planarian, for instance 

 Planar ia torva, transversely, the 

 posterior half, that has no brain, 

 crawls just as well as the oral half. 

 Spontaneity in Planaria torva is, 

 therefore, by no means a function 

 of the brain. Every piece of the 

 animal that is not too small pos- 

 sesses spontaneity. The decap- 

 itated animals crawl with the 

 anterior end in front like normal 

 animals (2). 



The question now arises as to 



1 FIG. 23. FRESH-WATER 



llOW It happens that 111 Thysano- PLANARIAN (I'LANA- 



zoon spontaneous movements cease RIA ToRVA )- 



r , 1 i 11 1 i '1 G, brain, n. longitudinal nerve. 



it the head be amputated, while in (After jijima.) 



fresh-water Planarians this opera- 

 tion does not have such a result. One is tempted 

 to account for the difference by the fact that the 

 fresh-water Planarians have more ganglion-cells 

 throughout the longitudinal nerves than the Thysano- 

 zoon. With the aid of comparative physiology it is 

 possible to show that such a view is untenable. In 



