Ej-es of Molluscs and Arthropods. 563 



around each of these central cells, or retinophorae, is arranged a 

 uumbcr of pigmeuted ones, — in tliis case more tban a single circle, — 

 wliieh also support nerve-bearing, cuticular rods (PI, 30, fig. 49). 



Such circles of cells, or ommatidia, form groups varying greatly 

 among themselves , either in the niimber of ommatidia, or in the pre- 

 cisiou with which they are separated from the surrounding epithelium. 

 A very common form of invaginate eye is that in which from two to 

 four or more closely placed ommatidia form a central, dark area, sur- 

 rounded by a zone of lighter colored pigment cells, in which are several, 

 irregularly scattered ommatidia, decreasing in niimber towards the 

 periphery of the band. In surface preparations, one may easily re- 

 cognize these scattered ommatidia by the color of the retinulae sur- 

 rounding the retinophorae. Although these isolated ommatidia are 

 usually Seen in greater abundance about the invaginated eyes, they are 

 often found quite independent of them scattered irregularly over the 

 Ophthalmie fold, and even in the other portions of the mantle. The 

 cuticula over these isolated retinulae, does not seem to be espeeially 

 thickened. 



Pseudo-leuticulate Eyes. 



I have found an interesting transitional form of eye, which belongs 

 neither to the invaginated nor faceted type, and which , for lack of a 

 better name, I shall call the pseudo-lenticulate eye. In sec- 

 tions, for instance, through that portion of the mantle where the invag- 

 inated eyes are most abundaut, I have found quite sharply defiued 

 groups of non-invaginated ommatidia , provided with a prominent len- 

 ticular thickeuing of the cuticula, containing vertical fibres undoubt- 

 edly of the same nature, i. e. nerve fibres, as those found in the cuti- 

 cular layer of the invaginated eyes (Fl. 30, fig. 54). 



A retinal cuticula seems to be formed by the pigmented cover cells, 

 as well as by the retinophorae, and heuce, in this respect, these eyes 

 resemble more closely the invaginated ones. On the other band, these 

 intermediate forms, which greatly resemble the ocelli oftheCoelenterata, 

 ofifer some similarity to the faceted eyes, in that they tend to form a 

 protuberant, convex surface, instead of a concave , invaginated one. 



Just as the invaginated eyes, composed of a varying number of 

 ommatidia closely or loosely arranged in groups, show all grades of in- 

 vagination, so may one find a parallel series of changes in the pseudo- 

 lenticulate eyes, together with all grades of convexity. In the faceted 



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