274 NERVOUS SYSTEM. 



oesophageal ganglia become subsequently separated off. The 

 observations on the pedal ganglia are less precise : they very 

 probably arise as thickenings of the epiblast of the side of the 

 foot. 



According to Fol, the nervous system in the Hyaleacea amongst the 

 Pteropoda originates in a somewhat different way. A disc-like area appears 

 in the centre of the velum, which soon becomes nearly divided into two 

 halves. From each of these there is formed by invagination a small sack. 

 The axes of invagination of the two sacks meet at an angle on the surface. 

 The cavities of the sacks become obliterated ; the sacks themselves become 

 detached from the surface, fuse in the middle line, and come to lie astride of 

 the oesophagus. Fol has detected a similar process in Limax. The exact 

 origin of the pedal ganglia was not observed, but Fol is inclined to believe 

 that they develop from the mesoblast of the foot. 



A very different view is held by Bobretzky (No. 242), whose observations 

 were made by means of sections. 



The supra- cesophageal and pedal ganglia are formed according to this 

 author as independent and ill-defined local thickenings of cells which are 

 apparently mesoblastic. The two sets of ganglia appear nearly simultane- 

 ously, and later than the rudiments of the auditory and optic organs. 



In the Cephalopoda there seems to be but little doubt, as 

 first pointed out by Lankester, that the various ganglia originate 

 in what is apparently mesoblastic tissue. 



There is still very much requiring to be made out with 

 reference to their origin, unless details on this subject are given 

 in Bobretzky's Russian memoir. It would seem however that 

 each ganglion develops as an independent differentiation of the 

 mesoblast (unless the optic and cerebral ganglia are from the 

 first continuous) 1 . The corresponding ganglia of the two sides 

 become subsequently united and the various ganglia become 

 connected by their proper commissural cords. The ganglia are 

 shewn in figures 124, 126, and 127. 



In Lamellibranchiata the development of the nervous system 

 has not been worked out. 



The two points which are most striking in the development of the 

 nervous system of Mollusca are (i) the fact that in the Cephalopoda at any 

 rate it is developed from tissue apparently mesoblastic ; and (2) the fact that 

 the several ganglia frequently originate quite independently, and subse- 

 quently become connected. 



1 Ussow states that they are independent. 



