472 



MOLLUSCA. 



Charybdea amongst the Acraspeda a more highly differentiated eye is 

 present, with a lens formed of cells like the vertebrate eye. 



Mollusca. In a large number of the odontophorous Mollusca 

 eyes, innervated by the supracesophageal ganglia, are present 

 on the dorsal side of the head. These eyes exhibit very various 

 degrees of complexity, but are shewn both by their structure and 

 development to be modifications of a common prototype. 



The simplest type of eye is that found in the Nautilus, and 

 although the possibility of this eye being degenerated must be 

 borne in mind, it is at the same time very interesting to note 

 (Hensen) that it retains permanently the early embryonic struc- 

 ture of the eyes of the other groups. 



It has (fig. 277 A) the form of a vesicle, with a small opening 

 in the outer wall, placing the cavity of the vesicle in free com- 

 munication with the exterior. The cells lining the posterior face 

 of the vesicle form a retina (7?); and are continuous with the 

 fibres of the optic nerve (N.op). We have no knowledge of the 

 development of this eye. 



In the Gasteropods the eye (fig. 277 B) has the form of a 

 closed vesicle: the cells lining the inner side form the retina, 

 while the outer wall of the vesicle constitutes the cornea. A 



N.op 



G.op 



FIG. 277. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA. 



(After Grenacher.) 

 A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod. 



Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; Int, Int" 1 ... 

 Int*. different parts of the integument; /. lens; I 1 , 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. 



