654 William Patten 



such structures known to exist. Even in one geniis, as Area, one finds 

 innumerable examples of the diffuse, the invaginate, and the evaginate, 

 or compound eye. 



The isolated ommatidia, constituting the diffuse form, seem to be 

 present in nearly ali of the Lamellibranchiata, regardless of whether 

 other and more perfect eyes are present or not. In the Gasteropods, 

 their presence has never been demonstrated , but from the rudiment- 

 ary character of some of their eyes [Patella, Helix, Haliotis, etc.), 

 there can be no doubt , that in the well developed pigmented areas of 

 these animals. there are also isolated ommatidia similar to those of the 

 Lamellibranchiata. 



The chromatophores of the Cephalopods offer 8triking»resemblauce8 

 to the isolated ommatidia. Blanchard (70) has shown that they develop 

 as single, eolorless cells surrounded by pigmented ones. If we suppose 

 that these embryonic organs are ommatidia, and that the colored cells, 

 or retinulae, should assume the features so characteristic of the pigment 

 cells in certain Coelenterata , Crustacea, andembryo fishes, i. e. amoe- 

 boid arms and contractility , the ommatidia would then be transformed 

 into chromatophores. Their originai function as ommatidia, and conse- 

 quently the intimate relation they bore to the light, would prepare the 

 way for their transformati on into their present condition. In the La- 

 mellibranchiata, the isolated ommatidia are found principally in those 

 pigmented areas exposed to the direct rays of light, that is, on the edge 

 of the mantle and tip of the sipho. In the former region , on the bran- 

 chial side of the Ophthalmie fold, the ommatidia tend to form intogroups, 

 covered with a slightly thickened cuticula, on the surface of rounded 

 summits or elongated ridges , or upon the floor of shallow grooves or 

 circular pits. Although in these cases the cuticula shows no traces of 

 a division for each underlying celi , it may be divided into two contin- 

 uouslayers; avery thin, structureless, outerone, or corneal cuticula, 

 and a thicker, less compact, inner one, the retinidial cuticula, filled 

 with the tibrillac of the retia terminalia (PI. 32, ^^. 130). 



In general, the ommatidia seem to be scattered about indefiuitely 

 over the pigmented surface of the mantle, but the irregulär folds in the 

 epithelium offer certain advantages which are seized upon by the om- 

 matidia, causing them to develop more rapidly in those regions with 

 the most folds. 



The invaginated eyes of ^4/r« may easily be proven to have origin- 

 ated from the pigmented furrows, or pits. Did the faceted, or evaginate 

 eyes , originate from the pigmented summits and ridges, or by a modi- 



