352 SEA -SIDE STUDIES, 



it is very questionable whether these are nervous in struc- 

 ture, and, if nervous, they are still only conducting-threads, 

 insensible to the direct influence of light. They are held to 

 be analogous to the rods and cones of the vertebrate retina, 

 ■which, as we have seen, receive their stimulus from changes 

 in the pigment, 7iot directly from the light. It is thus, as 

 Leydig says, "in the Vertebrata the rods form the outermost 

 layer of the retina ; in the Invertebrata they form the inner- 

 most. Herewith is connected the fact, which at first seems 

 so surprising, that the choroidcal pigment lies in front of the 

 retina, therefore the contrary of what occurs in Vertebrata."* 

 In the blind Crustacea no pigment is present ; and in Albinos, 

 in whom the pigment is of lighter colour, vision is imperfect. 

 If we remember that, according to the hypothesis, light only 

 aSects the retina after changing the temperature of the pig- 

 ment, which change is communicated to the rods and cones, 

 and thence to the vesicular layer, there will be nothing para- 

 doxical in this inverse arrangement of the retina in Inverte- 

 brata ; in both, the process is essentially the same, and the 

 mere difference of position is not more than the difference of 

 the chain of ganglia, which in the Vertebrata is dorsal, and 

 in the Invertebrata ventral.-f 



Returning from this digression, and its surprises, to the 

 eyes of our Nudibranchs, we can have little doubt that their 



• Leydio : Hisiolofjie, p. 253. 



t Lest it should be supposed I have overlooked it, I will notice one serious 

 difficulty in the way of the hypothesis just expounded, namely, the existence 

 in some animals of a strongly reflecting membrane — the tapctum — between the 

 retina and pigment layer. I do not at all understand the way in which this 

 affects vision, either on the old or new hypothesis. 



