228 



OPTIC ORGAN. 



of epiblastic invaginations. May not these invaginations be really rudiments 

 of the eyes as well as of the ganglia ? Fol also, it is true, describes a 

 similar mode of origin for these ganglia in Limax. It would be interesting 

 to have further observations on this subject. The independent origin of 

 the pedal and supra-cesophageal ganglia finds its parallel amongst the 

 Chzetopoda. 



The supra-cesophageal ganglia appear always to develop within the 

 region of the velar area. This area corresponds with the prse-oral lobe of 

 the Chfetopod larva, at the apex of which is developed the supra-cesophageal 

 ganglion. Embryology thus confirms the results of Comparative Anatomy 

 in reference to the homology of these ganglia in the two groups. 



Optic organs. An eye is present in most Gasteropods and in 

 many larval Pteropods. Although its development has not been fully 

 worked out, yet it has clearly been shewn by Bobretzky and other 

 investigators that it originates as an involution of the epidermis, which 



Int* 



Co.ep C 



PaL 



B 



JV.OJf 



N.op 



FIG. 122. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA (After Grenadier). 

 A. Nautilus. B. Gasteropocl (Lirnax or Helix). C. Dibranchiate Cephalopod. 

 Pal. eyelid; Co. cornea; Co. ep. epithelium of ciliary body ; I;-, iris ; Int,Int l ...Int*. 

 different parts of the integument; /. lens; /'. outer segment of lens; R. retina; 

 N.op. optic nerve; G.op. optic ganglion ; x. inner layer of retina; N.S. nervous stratum 

 of retina. 



first forms a cup and eventually a closed vesicle. The posterior wall of 

 the vesicle gives rise to the retina, the anterior to the inner epithelium 

 of the cornea. The external epidermis becomes continued over the 

 outer surface of the vesicle. 



The lens is formed in the interior of the vesicle, probably as a 

 cuticular deposit, which increases by the addition of concentric layers. 

 Pigment becomes deposited between the cells of the retina. Fig. 

 122 B is a diagrammatic representation of the adult eye of a 

 Gasteropod. 



The Cephalopod eye is formed, as first shewn by Lankester, as a 

 pit in the epiblast round which a fold arises (fig. 123 A) and gradually 



1 For a fuller account of this subject the reader is referred to the chapter on ' 

 Development of the Eye.' 



