120 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



Division of the brain in the middle line that is, 

 separation of the two halves of the brain destroys the 

 geotropic reactions of the eyestalks. It is still more 

 remarkable that such animals no longer prefer to re- 

 main in the dark like normal animals. 



If the longitudinal commissures be divided between 

 the mouth-ganglia (subcesophageal ganglion) and the 

 ganglion of the chelae, all locomotor movements be- 

 come impossible, although the legs are not paralysed. 

 This is strange, because the subcesophageal ganglion 

 contains the segmental nerve-elements of the oral ap- 

 pendages but not those of the locomotor appendages. 

 In other Crustaceans,extirpation of the subcesophageal 

 ganglion has no such paralysing effect on the loco- 

 motor movements. It is impossible to tell at present 

 what causes the exceptional behaviour of Astacus in 

 this regard. I do not believe we are obliged to as- 

 sume that this is an instance of a deviation from the 

 laws of the segmental arrangement of the nerve-ele- 

 ments (centres) of the limbs. This is shown by the 

 fact that the legs of such an animal are not paralysed, 

 but are unceasingly occupied in cleaning the abdo- 

 men, the pedes spurii, or each other. Indeed, more 

 than that, " if we give one of the forceps of a loco- 

 motor appendage a piece of meat or paper, other 

 legs approach immediately, seize the meat, and carry 

 it to the mouth," in spite of the fact that all nervous 

 connection between the nerves of the mouth-organs 

 and the legs has been severed. It is true that the ap- 

 pendages around the mouth often refuse to accept 



