EYES IN PPXTEN AND ARCA 



191 



pop down into their burrows in an instant, and it is vain to 

 attempt to dig them out. ' How sensitive,' remarks Mr. W. 

 Anderson Smith, with reference to oysters,^ ' the creatures are to 

 the light above them ; the shadow [of the boat] as it passes over- 

 head is instantaneously noted, and, snap! the lijJS are firmly closed.' 

 Ocelli of Pecten. — In Pecten and Siwndylus the ocelli are 

 remarkably large and prominent, shining like precious stones, and 

 are placed along the two edges of the mantle so as to receive the 

 light when the shell gapes (Fig. 93). In Pecten opercularis, 

 jacohaeus, and maximus their number varies from 80 to 120. 

 In Si^ondylus gaederoipus, a very inequivalve shell, 60 have been 

 counted on the right or fixed valve, and 90 on the left or upper 

 valve. Each ocellus is con- 



nected, by means of its 

 optic nerve, with the large 

 circumpalleal nerve, and so 

 with the branchial gan- 

 glion. They possess a cornea, 

 lens, choroidea, and optic 

 nerve, and, according to 

 Hickson,^ bear a consider- 

 able resemblance to the vertebrate type of eye. In spite of this, 

 the power of vision in these genera does not appear at all superior 

 to that of other Pelecypoda. 



According to the elaborate investigations of Patten, the 

 ' eyes ' in Area occur upon the middle or ' ophthalmic ' fold of 

 the mantle-edge, which is thickened at the end to admit of their 



reception. Along 

 this is ranged a row 

 of dark brown spots 

 of various sizes, 

 wliich are larger at 

 the anterior and 

 posterior ends of the 

 mantle - edge, but 

 smaller and more 

 numerous towards 

 are many of them 



Fig. 93. — Pecten opercularis L. , showing the 

 ocelli on the two edges of the mantle. 



Fig. 94. — Compound eyes {c.c) of Area harhata L. ; m.l, 

 mantle fold ; omm, ommatidia. (After Patten.) 



the middle. These brown spots, or ' eyes 

 compound, being made up of the fusion of a number of 

 ^ Berulerloch, p. 136. ^ Quart. Journ. Micr. Sac. xx. p. 443. 



