584 William Patten 



There is no necessity for regarding some eyes as »exochromic«, 

 and others as »auto chr orni e« , since there is no evidence to show 

 that pigmented mesoderm cells have forced their way through the 

 basai membrane iuto the eye ; ueither can it be shown, as any induce- 

 ment for the supposition that there are two kiuds of pigmentife- 

 rous cells in the eye, that they owe their origin to two different 

 sourees. Lankester moreover carries this supposition to an ab- 

 surd degree, in stating that such mesodermic cells have waudered 

 through the basai membrane, forcing their way between the retinulae, 

 then through a second membrane, and finally between the cry stalline 

 cone cells, in order to surround them with pigmented ones in a manner 

 to correspond to the arrangement of the retinulae around the rhabdoms. 



Moreover the pigment cells are the most ancient Clements in the 

 eye, older even than the retinophorae, and their origin, as well as that 

 of the retinophorae, is always ectodermic. It follows, then, that I cannot 

 accept his division of eyes into exochromic, and autochromic, since ac- 

 cording to my views such a division is not consequent. 



It must be borne in mind that I have not attempted to draw any 

 inference, foundedupon the structure of the eyes, in regard to the ge- 

 netic relationship of the different groups constituting the Arthropods. 

 I have simply made what seemed to be a reasonable supposition as to 

 the primitive Arthropod eyes, and have attempted to show how far, 

 and in what direction, the eyes now known diverge from this ancestral 

 form. Whether this evidence will be of any phylogenetic value, I am 

 not prepared to say. A verdict can only be rendered after a careful 

 study and comparison of ali other evidence entitled to consideration. 



A superficial examination of the eyes of Copepods and Cladocera 

 shows at once a resemblance between the crystalline body, — not the 

 cuticular lens which is sometimes developed, — and the crystalline 

 cone of the compound eye. If this resemblance is more than a super- 

 ficial one, we might attribute to them a similar function. Our know- 

 ledge of the structure of these eyes is too vague to allow of any specu- 

 lations concerning them. But if it could be proved that the ommatidia 

 were separated from the cuticula by a layer of cells, then there would 

 be ground for believing that they represent modified invaginate eyes 

 in which the retineum was reduced to one, or a small number, of om- 

 matidia. On the other band, it is possible that they may represent 

 single, or small groups, of isolated ommatidia which, without Separation 

 from the primitive hypodermis, have reached their present condition. 



Considering the fact that the retinophorae in Mollusca are double, 



