576 William Patten 



and in no species is to be foimd an equally strong, corresponding diflfer- 

 ence in the sìze and arrangement of the eyes. I have made a great 

 many experiments with Pecten Jacohaeus ^ and have foimd that, when 

 turned upon the fiat side , after a few hours they invariably siicceeded 

 in turning over. Among the large number constantly kept in the Aqua- 

 rium at Naples, I have never seen one resting on the fiat valve. In other 

 species of Pecten , where there is no difference in the curvature , stili 

 there is a tendency to rest upon the right valve ; and then, also, one finds 

 a corresponding diiference in the development of the eyes, those on the 

 right valve being, if not always smaller, at least less numerous. In 

 Arca barbata as well as Arca Noae , an exactly similar condition is to 

 be found. 



In attempting to account for this difference between the two sides 

 of the mantle, the first thought suggested is that the animai has become 

 accustomed to He upon one side, so that one part is more exposed to the 

 light than the other; therefore the eyes would attain their greatest 

 development at that point. It is true that, when such a difference was 

 once started , the advantage given to one side of the mantle would lead 

 to a stili further differentiation of the two parts. But why is it that 

 animals, which are capable of resting upon either one side or the other, 

 and between which there is no apparent difference in size or curvature, 

 have more and better developed eyes upon the left, than upon the 

 right fold of the mantle? Why is it, for instance, that in Arca Noae, or 

 Arca barbata, the left half of the mantle is more richly supplied with 

 eyes, than the right, although the right, left, or ventral side may be turn- 

 ed downwards? In Pecten opercularis , P. varitis, and others, there 

 is a well marked difference between the mantle edges , but in a great 

 many cases it is difficult to observe any diversity in the color and 

 curvature of the two shell valves. It is probable that there is a tendency 

 for these species to rest more upon one side than the other, but this can- 

 not be so constantly the case as in Pecten Jacobaeus , otherwise , there 

 would be a greater, and more Constant diiference between the valves of 

 the shell. There are , therefore , two factors which must influence the 

 development of the eyes at certain points : the first is an unknown one, 

 by whose agency a greater development of the pigment and visual Or- 

 gans upon the left side is produced ; it is also possible that the same 

 factor might cause a tendency to rest upon the right side ; the second 

 moment, which could only come into play after the action of the first, 

 is the advantage gained by having one side nearer the light, and more 

 removed from the sediment of the bottom. The best example of this 



