M. M. METCALF ON THE EYES AND SUBNEURAL GLAND OF SALPA. 343 



tially the same thing, toward the core of the optic ridge), while their 

 innervated ends are near the surface. The light then traverses the whole 

 length of the rod cells before reaching the pigment ; this eye is therefore 

 "inverted." 1 In the eye of the solitary form of Cyclosalpa pinnata a 

 secondary shifting of the retina has taken place by which it is changed 

 from the dorsal to the inner surface of the optic ridge. (Compare Figs. 

 7 and 8, Plate LI.) In the solitary form of other species the same process 

 is carried still further, till the thin-walled ends of the rod cells lie toward 

 the brain, and their thick-walled ends and the pigment layer lie just 

 beneath the ectoderm (Fig. 1, Plate LII, and Fig. 14, Plate LIV). In 

 these species the eye of the solitary form is therefore secondarily non- 

 inverted. This change from the inverted to the non-inverted condition, 

 shown in the ontogeny, is probably of phylogenetic significance. 



In the ontogeny of the eye of the chain form we see a similar change 

 effected in a somewhat more complicated manner. While the developing 

 eye of the chain Cyclosalpa pinnata is still a disk of cells appressed to the 

 ectoderm and connected to the brain only by the nerve fibers entering its 

 mid-ventral point (Plate XLVIII, Fig. 1), we can distinguish the three 

 regions of the eye (e\ e", e'"), and can determine the position of the as yet 

 imperfectly developed histological elements. In the first region, e', the 

 rod cells lie next to the brain and the pigment layer next to the ectoderm. 

 This region of the retina is therefore non-inverted. In the second region, 

 e", the rod cells lie next the ectoderm and the pigment layer toward the 

 brain ; this portion of the retina is therefore inverted. In the later 

 reversal of the eye, illustrated by the figures on Plate XLVIII, the rela- 

 tion of the whole retina to the ectoderm (i. e., to the morphologically 

 exterior surface of the body) remains the same. The relation to the 

 direction of the light is, however, exactly reversed. The first region, e', 

 which was at first unin verted, becomes physiologically inverted ; the 

 second region, e", which was at first inverted, becomes physiologically 

 non-inverted. The changes in the third region of the retina, e'", are 



1 1 have before described the reversal by which the eye of the chain individuals 

 of nine species is turned bodily upside-down in the course of its ontogenetic develop- 

 ment. I use the word inverted to refer to something wholly different, namely, the 

 condition of an eye when the bacilli of its rod cells and its pigment layer are away 

 from the source of light, while the protoplasmic, innervated ends of the cells are 

 toward the light. This is the technical use of the word inverted and has no reference 

 to what I have termed the reversal of certain salpa eyes. If it were not for the well- 

 established technical usage it would be well to exchange the terms, but as it is, such 

 a course would lead only to confusion. 



