60 EVENINGS AT THE MICEOSCOPE. 



either beneath the margin of the solid shell, or, if pro- 

 jected, projected only in the smallest degree, and en- 

 dowed 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 pig- 

 ment which lines the sclerotic 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 neiwe-trunk, 

 which runs along the border of the mantle. Thus there 

 exists in each of these lustrous points every element 

 needful for the due performance of vision, though, prob- 

 ably, the impressions thus conveyed may be neither so 

 powerful, nor so distinct as those which are conveyed 

 by the eyes of vertebrate animals. They are, how- 

 ever, 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 recog- 

 nise as eyes the shining black extremities of the upper 

 pair of " horns " in the Garden Snail. And though 

 some naturalists have doubted, and even denied that the 

 tentacle was anything more than a very delicate organ 

 of touch, yet it has been abundantly proved by dissec- 



