342 PHYSIOLOGY OF THE INVERTEBRATA. 



The cerebral or supra-CBSophageal ganglion of tlie Mollusca 

 appears to have special functions. According to M. Vulpian,* 

 if this ganglion in Helix is removed, the animal survives the 

 operation four or five weeks, but remains completely motion- 

 less. On the other hand, the removal of the sub-cesophageal 

 ganglion kiHs the animal in twenty-four hours. Mechanical 

 or electrical stimulation of the supra-oesophageal ganglion of 

 the Mollusca produces little or no effect ; but with the sub- 

 cesophageal ganglion, both kinds of stimulation cause vigorous 

 muscular agitation. Electrical stimulation often causes the 

 heart to stop, in the state of diastole. Exactly the same 

 phenomenon occurs when electrical stimulation is applied to 

 the pneumogastric nerves in the Vertebrata.^ 



These facts would seem to confirm the theory of the 

 German school of evolutionists, who connect the genealogy of 

 the Vcrteirata with the Mollusca; but this theory has had 

 its day, and the latest embryological researches explain the 

 origin of the Vertebrate brain and spinal cord as the outcome 

 of the nervous system of the Artliropoda. The nervous 

 system of the acranial Vertebrates can be considered as a 

 coalescent ganglionic nervous system. The central nervous 

 system of Amphioxus (one of the acranial Vertebrates) is a 

 spinal cord with a series of ganglionic enlargements, each of 

 which corresponds with the origin of a pair of nerves. An 

 enlargement, which is comparable to the central ganglion of 

 the Arthropoda, terminates (anteriorly) the spinal cord of 

 the acranial Vertebrata. It does not perceptibly differ from 

 the others, but gives ofE five pairs of nerves, among which 

 are the optic and auditory nerves. The great difference 

 between the Arthropoda and the Vertebrata is the complete 

 absence in the latter of an oesophageal nervous ring; and 

 that the nerve-cord has a dorsal aspect in the Vertebrata, 



* Lefons sur la Physiologie Gindrale et Comparee du Systeme Nerveux, 



pp. 757-761- 



t From these investigations it is diflScult to decide whether the supra- 

 or the.sub-oesophageal ganglion represents the brain in the Mollusca. 



