MOLLUSCA : THEIR EYES. 53 



deservedly recognised in the projection of the bony ridge 

 over the human eye, which we call the brow, we surely 

 cannot fail to recognise, and admire it also, in the position 

 of these delicate organs, either beneath the margin of the 

 solid shell, or, if projected, projected only in the smallest 

 degree, and endowed with the power of retreating beneath 

 its barrier with the rapidity of thought on the least alarm. 



There can be no doubt that these points, numerous as 

 they are, are true eyes, endowed with the faculty of vision 

 in a well-developed degree. For when their structure is 

 carefully examined by the skilful anatomist, each is found 

 to be covered with the proper sclerotic * tunic, which 

 becomes a perfectly transparent cornea in front, and to 

 possess a coloured iris, — perforated with a well-defined 

 pupil, and connected with a layer of pigment which lines 

 the choroid tunic, — a crystalline lens, and a vitreous 

 humour for the due refraction of the rays of light, and a 

 retina in their focus, formed by an expansion of the optic 

 nerve, and fitted to receive the picture ; the sensation of 

 which is then conveyed by an optic nerve from each eye 

 to the common nerve-trunk which runs along the border 

 of the mantle. Thus there exists in each of these lus- 

 trous points every element needful for the due performance 

 of vision ; though, probably, the impressions thus con- 

 veyed may be neither so powerful nor so distinct as those 

 which are conveyed by the eyes of vertebrate animals. 

 They are, however, we may be sure, amply sufficient for 

 the wants of the pretty Scallop, and are fresh proofs of 

 the Divine wisdom and benevolence. 



We have been accustomed from childhood to recognise 

 as eyes the shining black extremities of the upper pair of 

 "horns" in the Garden Snail. And though some natu- 



* The ball of the eye is composed of three coats or tunics, the 

 sclerotic, choroid, and retina, and contains three humours, the aqueous, 

 vitreous, and crystalline. The cornea is the front part of the sclerotic, 

 the iris that of the choroid. 



