550 William Patten 



inated eyes are usually situateci in cross canals so that, in most cases, 

 the smaller ones appear like simple, pigmented pits in the floors of the 

 canals. It often happens that, when these eyes are examined from 

 above, in surface preparations , they appear like very black, round, 

 pigment spots, and it is diffìcult to believe they are really sunken be- 

 low the surface. But on the other band, if the openings of the pits be 

 closed, as is often the case, one sees a black, narrow, and slit-like 

 opening with the fainter outline of the retina beneath (PI. 30, fig. 47). 

 In the latter instance, it is hardly necessary to prove by means of sect- 

 ions that they are pigmented pits sunken below the surface; in fact they 

 are exactly the same as those simple, invaginated eyes in the teutacles 

 of Patella. The number of these eyes is something extraordinary, when 

 we consider that these retiring animals are already provided with about 

 250 very perfect eyes. I bave counted in Arca harhata., 51/2 cm long, 

 as many as 420 or 430 eyes on one side, and 440 or 450 on the other, 

 not including a number of very small ones, diffìcult to distiuguish from 

 ordinary pigment spots. 



There is stili a third form of eye, the p s e u d - 1 e n t i e u 1 a t e , re- 

 sembling the last type , but not invaginated , and consisting of a few 

 retinal cells (to be soon more accurately described) , covered with a len- 

 ticular and refractive body like a cornea, or lens. These forms, diffìcult 

 to recognize except in sections , are distributed irregularly among the 

 invaginated eyes, with which they are brought into dose relationship 

 by a number of intermediate forms. I estimate that there are about 

 a hundred such eyes in each mantle edge (fìg. 54). 



Here then we bave a genus of almost motionless and helpless ani- 

 mais whose complex of activities consists in hardly more than closing 

 the shell to avoid an enemy, or opening it to obtain nourishment, and 

 yet each of these lowly organized animals has 250 compound eyes, each 

 of which (as we shall see later) is apparently as complicated an organ as 

 the eye of such active and carnivorous Amphipods, as Gammarus or 

 Orchestia. It has 800 or 900 eyes like these of Patella^ and then about 

 200 simple and minute ocelli, making a sum total of about 1300 eyes for 

 each individuai, not including numerous, small groups of ommatidia, 

 too minute to be easily counted by means of an ordinary pocket lens. 



Historical. 



When we consider that these remarkable eyes , which even a ca- 

 sual observer could not fail to see, bave since 1844 been known to 



