THE RETINA OF INVERTEBRATA. 369 



or otherwise a perception of light must take place, in 

 a manner incompatible with our knowledge of the or- 

 dinary mode in which the retina is affected by lumin- 

 ous rays/' * True, but the ordinary mode of conceiv- 

 ing the process, we have just seen to be untenable. 

 When Von Siebold says that the " mysterious pheno- 

 menon 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 hypothetical " eye-specs " 

 which have been noticed in Infusoria ? It is that of a 

 pigment spot on sl 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 certain 

 processes do pass through the pigment from the retina, 

 it is very questionable whether these are nervous ioi 

 structure, and, if nervous, they are still only conduct- 



* Owen : Lectures on Comp. Anatomy, p, 585. But he confesses not 

 to have seen such perforations. I have tried in vain to discover any. 

 \n front of the retina there is a dehcate 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 httle support from it. 



+ Von Siebold: Comp. Anatomy, p. 284. Very imperfect our 

 knowledge is ; although on what evidence Professor Rymer Jones 

 {Animal 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 exhibits it very clearly. 



2 A 



