MOLLUSC A: THEIR EYES 69 



about to make its sudden and vigorous leaps, the gem- 

 meous points are so situated as just to project beyond the 

 margin of the shell. So that when we view the creature 

 perpendicularly as it lies, our eyes looking down on the 

 convexity of the upper valve, the minute eyes are seen, all 

 round its circumference, just, and but just, peeping from 

 under its edge. It is clear that this arrangement secures 

 to them the widest range of vision with the least possible 

 exposure. As Divine contrivance has been often most de- 

 servedly recognized in the projection of the bony ridge over 

 the human eye, which we call the brow, we surely cannot 

 fail to recognize, 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 colored iris perforated with a well-defined pupil, 

 and connected with a layer of pigment which lines the 

 sclerotic tunic a crystalline lens, and a vitreous humor 

 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 lustrous points 

 every element needful for the due performance of vision, 



