=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_, _jacobaeus_, 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 branchial ganglion. They possess a cornea, lens, choroidea, and optic nerve, and, according to Hickson,[298] bear a considerable 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. [Illustration: FIG. 93.--_Pecten opercularis_ L., showing the _ocelli_ on the two edges of the mantle.] [Illustration: FIG. 94.--Compound eyes (_c.e_) of _Arca barbata_ L.; _m.l_, mantle fold; _omm_, ommatidia. (After Patten.)] According to the elaborate investigations of Patten, the ‘eyes’ in _Arca_ 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, which are larger at the anterior and posterior ends of the mantle-edge, but smaller and more numerous towards the middle. These brown spots, or ‘eyes,’ are many of them compound, being made up of the fusion of a number of ommatidia (from 10 to 80) into one large round eye, which is generally elevated above the surface of the surrounding epithelium. Sometimes these eyes themselves tend to fuse together. In one specimen of _Arca Noae_, 133 of these faceted eyes were counted in one mantle border, and 102 in the other.