164 ORGANS OF VISION. 



expressly adapted for discerning objects in the water. In 

 the octopus, loligo, etc. the eye-ball is placed in an orbital 

 cavity, of which it occupies the anterior portion : it is 

 remarkable for being destitute of a cornea, the trans- 

 parent skin being continued over the eye, so as to 

 supply the place of that tunic in mammalia. Beneath 

 this skin is seated the crystalline lens, which is large 

 and very convex ; and posterior to this is the vitreous 

 humour in its own membrane ; between this vitreous 

 humour and the retina, or expansion of a large optic 

 ganglion, is a layer of black pigment, such as in mam- 

 malia is behind the retina ; and it is difficult to imagine 

 how thus placed before the retina, it can admit the rays 

 of light to that nervous membrane. It cannot, indeed, 

 be supposed that it does ; and the question arises. How 

 is vision effected, or is this so called retina truly so ? 

 That it is an expansion of a large nervous ganglion 

 there can be no doubt ; but it would seem, from the 

 observations of Professor Owen, that over the anterior 

 surface of this pigment a filmy membrane is spread ; and 

 it is not impossible that this, although no connexion has 

 been discovered between it and the nervous membrane, 

 or the optic ganglion, may be the true retina, and that 

 some connexion, not discovered, may exist between it 

 and the large nerve in question. There is no true iris ; 



