SENSE ORGANS. ,63 



we have described in vertebrates that no comparison 

 can be instituted. In them we find a different kind of 

 instrument and not a mere modification and simplifica- 

 tion of that already studied. We shall come back to 



these after completing 

 the comparison in the 

 case of other inverte- 

 brates. 



Mollusca : Ceph- 

 alopods. — The higher 

 cephalopods, such as 

 the squid and cuttlefish, 

 have large eyes and by 

 far the most perfect 

 below vertebrates (Fig. 

 101). Their instrumen- 

 tal structure is substan- 

 tially like that of ver- 



FlG. 101. — Nervous system of an argonaut, 

 showing; the eyes : eg, cephalic ganglion ; 

 og t the optic ganglion ; nig, brg, vg, the 

 ganglia of the mantle, the branchiae, and 

 the viscera, respectively ; B, eye. (After 

 Cuvier. ) 



Fig. 102. — The eye 

 of a snail on the 

 end of the ten- 

 tacle, magnified. 



tebrates and is fully as perfect as that of a fish. There 

 are, indeed, some very significant differences, especially 

 in the retina, but these will come in our discussion of the 

 evolution of the eye. 



In the gastropods the lens is wanting, the vitreous 



