MOLLUSCA : THEIR EYES. 51 



and clear, it lies on one side, its valves being separated 

 as we see them now, a quarter of an inch or so apart 

 allowing us to discern what is contained between them. 



Well, we see first a number of slender white pointed 

 threads, peeping out from each valve, and spreading on 

 all sides, waving hither and thither, groping, now contract- 

 ing, now expanding, with incessant but deliberate motion. 

 These are tentacles. If we trace them to their origin, we 

 find them attached to a fleshy sort of veil that lines each 

 valve to near its edge, and then abruptly falls at an angle 

 towards the opposite valve, where it meets a correspond- 

 ing veil. These two veils form the mantle. It is from 

 each of these that the tentacles spring ; and we discover 

 that there are four rows of these organs, one row set 

 along the angle, and one along the edge, of each veil. 



But as we peer among these slender threads, our atten- 

 tion is riveted by some tiny points that are seated near 

 their bases, which glitter like brilliant gems. They are 

 seen only in those rows of tentacles which spring from 

 the angles of the veils, and not in those which fringe their 

 edges. Even the unassisted sight can detect the gleam 

 and glitter of these little specks ; but it is only when we 

 bring the lens to bear upon them that we see all their 

 beauty. Then they look like diamonds or emeralds, each 

 set in a broad ring of dark red substance, which greatly 

 enhances their beauty. They are inserted into the mantle 

 in the line of the tentacles, alternating with them, yet 

 not with absolute regularity, for there are more tentacles 

 than gem-points ; they are about half as numerous again 

 as the radiating ridges of the shell. Some are much 

 larger and more prominent than others, but they have 

 all the same structure and appearance. 



These little organs are eyes. As its movements are far 

 more extensive, and more fitful and rapid than is common 

 in this class of animals, the little Pecten probably needs 

 these brilliant organs of vision to guide its wayward 



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