THE RETINA OF IN VERTEBRATA. 351 



exposed to the light, yet in every case the vesicular and 

 granular layers and the optic fibres are beneath the pigment. 

 In the eye of the Cephalopoda this position of the pigment 

 has long been a puzzle, and Professor Owen says that it must 

 doubtless be " perforated by the retinal papilloB, or otherwise 

 a perception of light must take place, in a manner incompa- 

 tible with our knowledge of the ordinary mode in which the 

 retina is affected by luminous rays."* True, but the ordi- 

 nary mode of conceiving the process, we have just seen to be 

 untenable. When Von Siebold says that the " mysterious 

 phenomenon rests only on an imperfect knowledge of the 

 structure of the organ, "-f- he seems to me to forget that the 

 phenomenon is by no means peculiar to the Cephalopoda, but 

 is characteristic of the Invertebrata generally. What, for 

 instance, is the simplest form of an eye, disregarding those 

 hjrpothetical " eye-specs " which have been noticed in Infu- 

 soria? It is that of a pigment spot on a ganglion, or a 

 nervous expansion. Ascending higher in the scale, and 

 reaching even the complex structure of the crab's eye, what 

 do we find but a pigment layer covering the retina ? If cer- 

 tain processes do jDass through the pigment from the retina, 



* Owen : Lectures on C'omp. A natomj/, p. 585. But ho confesses not to liavc 

 seen such perforations. I have tried in vain to discover any. \ii front of tlio 

 retina there is a delicate membrane, but it has none of the characteristics of a 

 nervous tissue, nor have I been able to trace any communication between it and 

 the retina, through the pigment. Even should such a communication exist, 

 the ordinary theory of vision would derive little support from it. 



•\ Von Siebold : Comp. Anatonii/, p. 284. Verj' imperfect our knowledge 

 is ; although on what evidence Professor Eymcr Jones (.1 tiivuil Kingdom, 

 p. 591) denies the existence of the choroid, I know not. I have not only seen 

 it repeatedly, but have made a preparation which exliibits it very clearly. 



