MOLLUSC A. 227 







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 invagiiiation a 

 small sack. The axes of invaginatioii of the two sacks meet at an angle on 

 the surface. The cavities of the sacks become obliterated ; the sacks them- 

 selves 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 observa- 

 tions were made by means of sections. 



The supra-o3sophageal and pedal ganglia are formed according to this 

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

 apparently inesoblastic. The two sets of ganglia appear nearly simul- 

 taneously, 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 (1) 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 

 subsequently become connected. 



With reference to the first of these points it should be noticed that 

 the supra- oasophageal and pedal ganglia are at first respectively connected 

 with the optic and auditory organs, and that these sense organs are in 

 some cases at any rate developed anteriorly in point of time to the ganglia. 

 It seems perhaps not impossible that primitively the ganglia may have been 

 simply differentiations of the walls of the sense organ, and perhaps their 

 apparent derivation from the mesoblast is really a derivation from cells 

 which primitively belonged to the walls of these sense organs. Bobretzky's 

 observations on Fusus fit in well with this view. 



In the Hyaleacea and in other Pteropods, where the eyes are absent 

 in the adult, Fol finds the supra-cesophageal ganglia resulting fiom a pair 



1 Ussow states that they are independent. 



152 



