THE NERVOUS SYSTEM. 



301 



the sea water. Another fold round the eye gives rise to the eye- 

 lid found in some Cephalopoda (especially in the Octopoda, Fig. 145 



0, Inf 4 ). 



Int* 



JV.op 



G J? N.op 



FIG. 145. Diagrams representing the eyes of Nautilus (A], a Gastropod (/i), and one 



slayer of the retina ; /W, eyelid ; R, retina ; x, outer layer of the retina. 



The course of development of the Cephalopod eye described above 

 shows that it attains a high degree of perfection, as may indeed be 

 seen from an examination of the adult eye. 



D. The Nervous System. 



All investigators of the origin of the nervous system in the Cephalo- 

 poda were formerly unanimous in tracing it back to the mesoderm 

 (KAY LANKESTER, Ussow, BOBRETZKY). More recently, the forma- 

 tion of the nervous system has been studied by VIALLETON, who found 

 that the ganglia arise through the thickening of the external layer ; 

 these thickenings, however, yield at the same time mesodermal tissue. 

 Other similar thickenings of the ectoderm are principally formative 

 centres for the mesoderm.* This view also, therefore, does not 

 establish any sharp distinction between the central nervous system 

 in the process of formation and the mesoderm. At the same 

 time, we have to emphasise the fact that in the Cephalopoda as in 

 other Molluscs, the nervous system is of purely ectodermal origin 



(KORSCHELT, No. 25). 



* See further, p. 307. 



