Dr. B. Hicks on the Homologies of the Eye in the Invertehrata. 323 



The homology of the nerve-sheath is traced from its condition aa 

 a delicate membrane which passes over the simple eye, to that of the 

 sclerotic and cornea ; and it is shown to constitute the true cornea. 

 After referring to the pigment and iris, the author discusses the 

 parts added to the simple eye. The anterior chamber, or the space 

 found anteriorly to the lens and iris (where the latter exists), first 

 appears in the Leeches, but is a very variable element in the Inver- 

 tehrata, existing in some aquatic animals, as Leeches and Cephalous 

 MoUusks, while it is absent in others, as Paludina, Limncea, &c. ; 

 and thus the opinion held by some, that an anterior chamber is of 

 little use in aquatic animals, as in Fishes, is scarcely tenable, because 

 they have a compensation in the globular lens. The author has 

 never found an anterior chamber in the compound eyes of the In- 

 secta, although a space has been described, and erroneously called a 

 lens. Still the possibility of such a space is not thereby denied ; 

 only, should it exist, it would be homologous to the anterior chamber, 

 and not to the lens ; and the transparent body behind it is the true 

 lens, and not a vitreous humour, as is often represented. There is 

 such a chamber in the Decapods, in which it is well marked. In 

 regard to a chamber posterior to the lens, the author is of opinion 

 that none is to be found below the Mollusks, and points out that what 

 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- 

 hrata 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, in Artemia 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 

 crystalhne 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 compUcated 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 



