48 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The larger dorsal eye is seen in figure 3 1 , plate 1 3 , and figure 1 9, on this 

 page (copied from Metcalf and Johnson, 1 905) . The section shows that 



Fig. 19.— Cyclosalpa virgula, aggregated form, a sagittal section of the ganglion 

 and dorsal eye. From Metcalf and Johnson (1905). 



the eye is divided into two main portions; a basal portion (e') in which 

 the rod-cells are dorsal to the pigment, and a terminal portion (e") 



in which the rod cells are ventral to the 

 pigment. A little behind the mid-dorsal 

 point of this eye there is an area of 

 thickened pigment partially surround- 

 ing a group of larger cells which are the 

 remnant of the optic plug {e'" , see also 

 fig. 20) which in Cyclosalpa pinnata and 

 •4- IJ'f'-fN'^fAM C. affinis is much better developed. In 

 lMi7\\Am^:^%B!^^ C.floridana and C. bakeri the optic plug 

 becomes more closely united to the apical 

 portion of the eye. In C. virgula there 

 is but a vestige of this structure. Tho 

 vestigial condition of its optic plug shows 

 that, in the character of its large dorsal 

 eye, C. virgula is intermediate between 

 the more archaic Cyclosalpas on the one hand and the Apsteinias, 

 Salpas, and other subgenera on the other. The optic nerve enters 

 the eye passing above the basal rod cells (fig. 19). 



Fio. 20.— Cyclosalpa virgula, aggre 

 gated form, the middle portion of 

 the section of the dorsal eye 



SHOWN IN FIG. 19, MORE niGHLY MAG- 

 NIFIED. From Metcalf and Johnson 

 (1905). 



