78 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



the crayfish, the suboesophageal ganglion with the 

 ventral ganglion chain represents a much more highly 

 developed ganglion-system than the longitudinal 

 nerves in Planar ia torva. We shall see, nevertheless, 

 that a crayfish, which possesses these ganglia, but 

 has lost the supracesophageal ganglion, no longer 

 moves spontaneously. We shall see, furthermore, 

 that a frog that has lost the cerebral hemispheres and 

 thalamus opticus does not move spontaneously, al- 

 though it possesses many more ganglia in the spinal 

 cord than Planar ia torva. The same frog, however, 

 moves spontaneously again if, in addition, the optic 

 lobes and the pars commissuralis of the medulla 

 oblongata be removed. 



Spontaneous progressive movements are not a 

 specific function of ganglia or of ganglion-cells ; we 

 observe them even in the swarmspores of algae and 

 in bacteria. Why the decapitated Thysanozoon no 

 longer performs progressive movements, and a decap- 

 itated fresh-water Planarian continues to move spon- 

 taneously, we are not yet prepared to say. It is 

 possible that the difference between fresh-water and 

 marine Planarians is somewhat of the same character 

 as that between Hydromedusae and Acalephae. In 

 the latter, both parts, margin and centre, beat rhyth- 

 mically in sea-water, while in the Hydromedusae only 

 the margin with the nerve-ring is able to do so. But 

 we were able to show that this difference between the 

 two classes of Medusae is not so much due to mor- 

 phological differences as to chemical or physical 



