81 



January 17, 1861. 



Major- General SABINE, R.A., Treasurer and Vice-President, 

 in the Chair. 



The following communication was read : 



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

 Invertebrata." By J. BRAXTON HICKS, M.D. Lond., 

 F.L.S. Communicated by JOHN LUBBOCK, Esq. Re- 

 ceived December 20, 1860. 



(Abstract.) 



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 diiferent 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 ; Srdly, 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 



