83 



has been so called in the ocelli and stemmata is not the homologue 

 of the vitreous humour, but probably that of the lens, which other- 

 wise is absent in these eyes, for the lens of the ocelli is shown below 

 to be of dermal origin. 



The next portion of the paper is occupied in pointing out the 

 part which the integument plays in regard to the optical organs 

 where it passes over them ; and it is shown that in all of the Inverte- 

 brata the integument becomes more or less subservient to the func- 

 tions of those organs. It is in animals with a dermo-skeleton that 

 the greatest subjection of the integument to the eye is seen ; and 

 the change is reviewed gradatim from where the eye is independent 

 of the integument, as in Daphnia, through Gammarus, where the 

 integument is merely applied to it, till, mArtemia Salina, we find the 

 inner surface indented, but not yet facetted. In Branchipus the 

 inner layer only becomes facetted, till in the Decapods both layers 

 become so. But it is in the Insecta that the change is most marked, 

 the form of the facets becoming more or less lens-like, till in Diptera 

 they are nearly perfect spheres. Their power of refraction is very 

 high ; they are supplied with corrections for spherical and chromatic 

 aberration, and they almost altogether supply the place of the true 

 crystalline lens, as before pointed out. The ocelli in Insects and 

 Arachnida show this condition carried to the extreme ; and the 

 author conceives that the so-called crystalline lens of these organs is 

 derived entirely from the integument : this he endeavours to show 

 by reference to their structure and intimate connexion with the other 

 dermal layers, and by tracing them through the various tribes ; and 

 a more complicated structure is described, for the first time, in 

 the median eye of one of the Scorpionidse (Buthus), where the inner 

 integumental layer is converted into a lens more distinct than in any 

 other of the Arachnida. A chamber is also shown to exist in it, 

 formed by the separation of the epidermis from the other layers, 

 producing a plano-convex lens similar in form to the anterior 

 chamber of the higher animals, and analogous to it in function. 

 The stemmata and ocelli, like the facets of the compound eye, have 

 peculiarities in their structure for the correction of spherical ajid 

 chromatic aberration. 



The author concludes by pointing out the striking homology 

 existing throughout the whole animal world in those parts which 



