No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 177 



ing down the surface of the cell in lines of scarcely discernible 

 material, suggesting processes of minute pigment cells, but 

 regarded as shreds of the superficial pigmented portion of the 

 large retinal cell. 



In such preparations the lens is analyzed into a mass of ice- 

 like fragments, which, however, as shown in Fig. 7, do not agree 

 in size with the ends of the retinal cells. They are also not as 

 numerous as those cells.^ 



The eyes in the large atoke individuals, found amongst algae 

 upon buoys, etc., have the same structures as in the epitoke 

 condition. The same is true of small individuals, 4 to 6 mm. 

 long (Fig. 5), taken in such places, or in the tow net. Yet 

 here the eye is more simple and easily understood (Fig. 21). 

 The number of retinal cells is small, the pigment not abundant, 

 the cornea evidently part of the epidermis converging over the 

 lens. As the blue pigment is restricted to the dense zone, 

 the bright yellow is less concealed, and the entire eye has a 

 golden instead of a red color, as seen from the surface. 



As the retinal cells separate more readily than in the adult, 

 their character is beautifully and easily shown in macerations 

 (Fig. 20). 



Nereis virens Sars. 



Only atoke, immature, but large specimens were studied, 

 chiefly by section methods. 



In all parts the four eyes have the same structure as that 

 described for the two preceding species. 



The eyes are far less conspicuous, owing to the thickness of 

 the cuticle and epidermis, and to the small size of the outer 

 lobe of the lens (Fig. 3). The cornea is composed of much 

 elongated epidermal cells, converging over the conical outer 



^ The size of the pupil varies even more than in the previous species. The difference 

 is not a sexual one : one specimen has one pupil remarkably small, and the other 

 three large. That the size is constant in the individual is not proven; on the con- 

 trary, the following experiment furnishes some slight reason to judge there is a change 

 in connection with illumination. Nine individuals kept 36 hours in total darkness 

 show an average diameter of 1.5 in 12 eyes measured, as compared to I for the same 

 in 9 kept in daylight and gaslight. The experiment was somewhat vitiated by unequal 

 conditions of temperature and purity of water, and great actual variations in size of 

 the pupils were noted. Those in the dark ranged from 112 /u to 182 /a; those in 

 light, from 42 /:i to 168 /t ! 



