176 ANDREWS. [Vol. VII. 



Cornea. — This name may be given to the thin layer of epi- 

 dermis intervening between the lens and the cuticle (Fig. 8). 



Though in some sections this layer seems absent, yet surface 

 views show the epidermal nuclei scattered all over the area 

 external to the lens. This epidermal layer is, however, very 

 thin over the centre of the lens, consisting of but a film of 

 protoplasm containing scattered nuclei. 



The cuticle over this region might be included as part of the 

 cornea. It is, however, not modified in any way, and is directly 

 continuous with the similar cuticle over the neighboring parts 

 of the head. 



There is but little elevation of the surface over the eye, that 

 indicated in the figure being due to the situation of this eye at 

 the angle between the dorsal and lateral aspects of the head.^ 



Nereis limbata Ehlers. 



The mature epitoke state of this species is found with that of 

 the former species, and equally, or even more abundantly. 



The four eyes have the normal size and position (Fig. 11). 

 The two anterior ones are somewhat the larger, and look forward 

 and outward, nearly horizontally. The posterior eyes look back- 

 ward, outward, and markedly upward. Thus the four visual 

 axes tend to converge, when produced, towards some point 

 within the brain. Owing to this inclination of the axes, it is 

 almost impossible to obtain a section through the visual axes of 

 two eyes simultaneously ; yet an obliquely transverse plane 

 cutting anterior and posterior eyes on opposite sides of the head 

 would nearly do this. 



The structure of all the eyes is the same, and identical with 

 that just described in N. alacris. This identity extends to the 

 character of the blue-red pigment and to the yellow pigment, 

 to the arrangement of these in the nucleated retinal cells, as 

 well as to the general structure of the retinal cups, layer of 

 rods, lens, and cornea. 



Macerated retinal cells (Fig. 4) show the dark pigment extend- 



^ As bearing upon the function, and possibly upon the evolution, of the enormous 

 sexual eyes, it is interesting to note that the males are greatly in excess, amongst the 

 individuals taken at the surface, and rarely present a perfect set of tentacles, even 

 upon one side of the head. Some dorsal ones are broken off, leaving the basal joint. 

 Is this due to a struggle amongst the males, or may the jaws be used in grasping 

 during fertilization? 



