564 William Patten 



eyes, the ommatidia, whether single or uniteci in groups, show the same 

 eomposition. It is rather remarkable that the ommatidia of each type 

 of eye should retain so eonstantly their specific characters, even in 

 cases where the two types which they produce are so little different- 

 iated from each other. I believe that, in both cases, the ommatidia are 

 different phases of the same structural Clements , those of the faceted 

 eyes being undoubtedly of a higher order than those of the invaginated 

 ones, the latter being almost identical in structure with those isolated 

 ommatidia , fouud irregularly distributed over the exposed portions of 

 the mantle. The isolated , faceted ommatidia are only found — and 

 then rarely — in the immediate vicinity of what appear to be degener- 

 ate evaginate eyes. It is more probable that the higher structure of the 

 faceted ommatidia vras attained, after they were united into distinct 

 groups, than when in the isolated condition. If this is the case, we 

 should expect to find in those imperfectly formed faceted eyes , the pe- 

 ripheral ommatidia in a less perfect stage of development, inproportion 

 as they were more distant from the centre of the eye. But this is 

 not the casC; for I bave often seen isolated ommatidia on the peri- 

 phery, as highly developed as any ever found in the centre. For 

 this reason, I consider such eyes as retrogressive, rather than pro- 

 gressive , and possibly analogous with those cases in PecAen , where 

 fully formed eyes become functionless, by the complete pigmentation of 

 the cornea. 



It is probable that during the phylogenetic development of the vi- 

 sual organs of Arca , those portions of the mantle, most exposed to 

 light, developed pigmented cells , among which were numerous color- 

 less (gland?) cells. By the fusion of two such pigmentless cells, and 

 the accumulation around them of a definite number of pigmented ones, 

 the isolated ommatidia were formed ; they in turn united into special 

 groups, where the ommatidia became modified in certain directions 

 according to the form and position of the organs they composed. The 

 number of the isolated ommatidia must bave been , at first, very great^ 

 but probably diminished, as they accumulated to form more complex 

 organs of greater functional activity; after the formation of these 

 groups, the development of the ommatidia continued, until an enor- 

 mous number of visual organs was formed, in a high stage of perfection, 

 and of a functional activity disproportionate with the requirements of 

 the animai; a tendency for certain of the eyes to degenerate would 

 therefore arise. Such is at present the condition in Aì^ca and Pecten. 

 It is probable that in both cases, peculiar, favorable conditions were 



