EYES IN PECTEN AND ARCA 



191 



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

 tempt to dig them out. ' How sensitive,' remarks Mr. W. Ander- 

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

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

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



Ocelli of Pecten. — In Pecten and Spondylus 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, jacohaeits^ and maximus their number varies from 

 80 to 120. In Spondylus gaederopus, 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 connected, by 

 means of its optic nerve, 

 with the large circumpalleal 

 nerve, and so with the bran- 

 chial ganglion. They pos- 

 sess a cornea, lens, choroidea, 

 and optic nerve, and, accord- Fig. 93 

 ing to Hickson,2 bear a con- 

 siderable 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 



7 of various sizes, 



which are larger at 



the anterior and 



posterior ends of the 



mantle - edge, but 



m.l, smaller and more 



towards 



Pecten opercularis L., showing the 

 ocelli on the two edges of the mantle. 



Fig. 9i. — Compound eyes (c.e) of Area harhata L. ; 

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



numerous 



the middle. These brown spots, or 'eyes,' are many of them 



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



^ Benderloch, p. 136. 2 Quart. Journ. Micr. Sac. xx. p. 443. 



