342 PHYSIOLOGY OF THE INVERTEBRATA. 



The cerebral or supra-ccsopliageal ganglion of the Mollusca 

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

 if this ganglion in Hdiv 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- oesophageal 

 ganglion kills the animal in twenty-four hours. Mechanical 

 or electrical stimulation of the supra-cesopliageal ganglion of 

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

 oesophageal 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 Vcrtehrata.] 



These facts would seem to confirm the theory of the 

 German school of evolutionists, who connect the genealogy of 

 tbe Vcrtcbrata with the Mollusca ; but this theory has had 

 its day, and the latest embryological researches explain the 

 origin of the Vei-tebrate brain and spinal cord as the outcome 

 of the nervous system of the Arthropoda. The nervous 

 system of the acranial Vertebrates can be considered as a 

 coalescent ganglionic nervous system. The central nervous 

 system of Ariiphioxus (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 

 tlie Arthropoda, terminates (anteriorly) the spinal cord of 

 the acranial Vcrtchnda. It does not perceptibly differ from 

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

 are the optic and auditory nerves. The great difference 

 between the ArthTop)oda and the Vcrtehraia is the complete 

 absence in the latter of an oesophageal nervous ring ; and 

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



* Lerons sur la I'liijsiolof/ic Genc'ride et Compare'e iJii /Systaiie Kerveux, 



pp. 757-761. 



t From these investigations it is difficult to decide whetlier tlie supra- 

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



