200 



THE EYE IN EVOLUTION 



Avicula 



Mi/tilus 



Pholas 



Cardium 



illumination above and below are probably appreciated through the dorsal and 

 ventral " windows " (Hesse, 1908 ; v. Hess and Gerwerzhagen, 1914). Such 

 fenestrated eyes are also seen in abyssal fishes.^ 



LAMELLiBRANCHS or BIVALVES have ail Undeveloped head-region, 

 and the two lobes of the mantle which secrete the two valves of the 

 shell are frequently united posteriorly to form exhalant and inhalant 

 siphons. Anterior eyes are therefore rare. Such cephalic eyes are 

 sometimes seen in larval forms but in the adult they tend to become 

 vestigial remnants, a cupulate depression of bipolar sensory and 



>..7.J-.i_ 



X--^'y^^^ 



Fig. 190. — The Common Scallop, Pectes. 



The pallial ocelli, Oc, are seen in a single row i-ound the margin of the 

 mantle. For section of the eye, see Fig. 123 (after Pelseneer). 



pigmented cells as occurs in the j^earl-oyster, Avicula, or the edible 

 mussel, Mytilus. More usually they are replaced by ocelli located in 

 situations where they are of greater biological value such as the 

 siphons, the tentacles or the mantle (Fig. 190). 



Thus the ocelli are found on the inner surface of the siphons in clams which 

 habitually lie buried in the sand or mud (Mya) or bore into soft rocks (Pholas) 

 (Light, 1930) ; as they lie buried these molluscs extend the siphon to the surface 

 to feed and at daybreak or whenever the illumination increases the siphon is 

 withdrawn (Wenrich, 1916 ; Hecht, 1919-20 ; Pieron, 1925 ; Folger, 1927 ; 

 and others). It will be remembered that these visual organs are of the most 

 simple type resembling those of the earthworm, being merely single cells of the 

 apolar type with a refractive organelle in the cell-body richly supplied with 

 nerves.- In the cockle, Cardium,, small ocelli are situated at the tips of the 

 tentacles, about 100 in number, which are arranged around the siphonal 

 apertvires ; the eye is of a simple cupulate form, the cuj^-shaped retinal cells 

 resting on a layer of double pigmented cells underneath a large ectodermal 

 cell^iJar lens and cornea (Kishinouye, 1894). As in the pallial eyes of Pecten, 

 the !ct ina is inverted. 



Pecten 



1 p. 323. 



p. 131. 



