370 ' SEA-SIDE STUDIES. 



ing-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, not directly 

 from the light. It is thus, as Leydig says, " in the Ver- 

 tebrata the rods form the outermost layer of the retina ; 

 in the Invertebrata they form the innermost. Here- 

 with is connected the fact, which at first seems so sur- 

 prising, that the choroideal pigment lies in front of the 

 retina, therefore the contrary of what occurs in Verte- 

 brata." * In the blind Crustacea no pigment is pre- 

 sent ; and in Albinos, in whom the pigment is of 

 lighter colour, vision is imperfect. If we remember 

 that, according to the hypothesis, light only affects the 

 retina after changing the temperature of the pigment, 

 which change is communicated to the rods and cones, 

 and thence to the vesicular layer, there will be nothing 

 paradoxical in this inverse arrangement of the retina 

 in Invertebrata ; in both, the process is essentially the 

 same, and the mere difi'erence of position is not more 

 than the diff'erence of the chain of ganglia, which» in 

 the Vertebrata is dorsal, and in the Invertebrata 

 ventral. -[- 



Eeturning from this digression, and its surprises, to 



* Leydig : Hlstologie, p. 253. 



*f- Lest it should be supposed I have overlooked it, I will notice one 

 serious diflSculty in the way of the hypothesis just expounded, namely, 

 the existence in some animals of a strongly reflecting- membrane — the 

 tapetum — between the retina and pigxaent layer, I do not at all un- 

 derstand the way in which this affects vision, either on the old or new 

 hypothesis. 



