70 EVENINGS AT THE MICROSCOPE 



though, probably, the impressions thus conveyed may be 

 neither so powerful, nor so distinct, as those which are con- 

 veyed 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 wis- 

 dom and benevolence. 



We have been accustomed, from childhood, to recog- 

 nize 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 dissection, and is now 

 incontrovertibly established, that its tip carries an eye even 

 more completely developed than those of the Pecten, which 

 we have just been looking at. The eye is situated, not 

 indeed on the very summit of the tentacle, but on one side 

 of a movable bulb there placed. It is very minute, almost 

 spherical, but slightly flattened in front. It is protected 

 by a very thin transparent layer of the common skin, and 

 is surrounded, at the side and behind, by a, perfectly black 

 membrane called the choroid, or pigment membrane. This 

 black globule contains a transparent and semi-fluid sub- 

 stance, with which it is completely filled; toward the bot- 

 tom it is of thinner consistence, and appears to contain 

 many brilliant particles when the eye is dissected under 

 the microscope; this may be considered as the vitreous 

 humor. In the front part of the eye there is a crystalline 

 lens, a small, circular, flattish, or rather lenticular body, 

 perfectly clear and translucent, but a little more solid than 

 the vitreous humor. 



Now protection for these so delicate organs is provided 

 in a way quite different from, yet equally effective with, 



