822 Royal Society : — 



PROCEEDINGS OF LEARNED SOCIETIES. 



ROYAL SOCIETY. 



January 17, 1861. — Major-General Sabine, R.A., Treasurer and 

 V.P., in the Chair. 



" On the Homologies of the Eye and of its Parts in the Inverte- 

 brata," by J. Braxton Hicks, M.D. Lond., F.L.S. 



The author first remarks, that the great similarity which exists in 

 the parts of the eye throughout all classes of the Vertebrata, coupled 

 with the desire to find in the eyes of all animals the same component 

 parts as in that great group, has militated much against a proper 

 understanding of the different parts of the Invertebrate eye. 



A different method has been followed in the observations upon 

 which the present communication is founded ; for, starting from the 

 simplest condition in which the organ appears in the animal series, 

 the necessary elements of a picture-seeing eye are first determined, 

 and afterwards those parts which are superadded to it. A distinction 

 is then drawn between the mere light-seeing eye and that which 

 perceives a picture ; the former requiring no lens, the latter having 

 one. The elements of the picture-seeing eye are, in fact, — 1st, a 

 nerve-fibre, and bulb ; 2ndly, a cell possessing more or less refrac- 

 tive power, and resting on the nerve-bulb ; 3rdly, pigment ; 4thly, 

 the nerve-sheath, including the other structures. The homologies 

 of these parts are then considered, and traced through the various 

 classes. The cell resting on the nerve-bulb is the homologue of the 

 crystalline lens. It has the power of secreting into its interior a 

 highly refractive substance, which, in the Invertebrata generally, is 

 semifluid, but sometimes, as in certain Insecta, solid. The bilateral 

 tendency of the cell in Asterias and in some Entomostraca is pointed 

 out, as also the tendency of the solid lens of Insects to split longitu- 

 dinally into four portions. This is compared with the same con- 

 dition in the more fluid lens of the Decapod Crustaceans, and referred 

 to the mode of development as set forth by Dr. E. Claparede. More- 

 over, as in many cases these cells become so fused as to form an 

 entirely homogeneous body, the author thinks that the single 

 crystalline lens, possessing many nerve-bulbs (as in MoUusks), is 

 formed really by the combination of their various lenses ; the pig- 

 ment in this case being confined to the nerve-bulb, which is its 

 normal position. 



It is then pointed out, for the first time, that in some insects, and 

 in lulus, the lens possesses a very slight refractive power, being filled 

 with a fluid as little refractive as water. This is more particularly 

 the case in the Diptera, where the dermal facets become so spherical 

 as to usurp the function of the lens, while an example is furnished 

 by the eye of Aromia moschata of the complete disappearance of the 

 lens. In opposition to the opinion entertained by many naturalists, 

 the author maintains that each bulb, provided with a refracting body, 

 is capable of perceiving a picture independently of the assistance of 

 other adjoining lenses. 



