No. 2.] EYES OF POLYCH^TOUS ANNELIDS. 171 



down to a ventral position next the mouth, while the antennae no 

 longer project forward, but hang down vertically from the 

 peculiar anterior prolongation of the head. 



The general appearance of the eyes in life is indicated in 

 Fig. I. The large posterior pair are approximately spherical, 

 and have their visual axes directed dorsally, posteriorly, and 

 somewhat outward, so that when prolonged they would tend to 

 converge towards some point upon the anterior edge of the 

 mouth. The anterior eyes are much larger still, less regular in 

 shape, more elongated, and look downwards, so that the visual 

 axes are nearly vertical, but yet, diverging somewhat outwards, 

 would, if produced, tend to meet at some point dorsal to the 

 head. 



In this side view, both anterior and posterior eyes are well 

 shown, but in ventral view, only the elongated elliptical retinas 

 and circular pupils of the large anterior eyes are visible. In 

 dorsal view, the pupils of the smaller posterior eyes are seen 

 surrounded by the somewhat elliptical retinas, while the dorsal 

 surfaces of the anterior eyes are also visible. 



The lenses and corneas are seen only in profile views. 

 The size of the pupil varies much ; that of the anterior is 

 frequently greater than that of the posterior, but the reverse 

 may occur. Moreover, amongst specimens subjected to the 

 same treatment, some have much larger pupils than others, in 

 all, or in some of the four eyes. 



As commonly seen in life the eyes have a dark red color, 

 modified by a golden yellow tinge. The pupils, however, appear 

 dark blue. The explanation of this is evident when retinas are 

 teased out in sea-water. There are two kinds of retinal pig- 

 ment : the one, golden yellow, is less concealed in the superficial 

 or peripheral part of the retina, and gives the golden reflections 

 from its surface ; the other, in mass, or in separate granules, is 

 red by reflected, and dark blue by transmitted, light, and espe- 

 pecially predominant in the part of the retina turned toward the 

 centre of the eye. Hence the red and yellow light reflected 

 from the eye, the blue light transmitted through the retina and 

 emerging from the pupil. 



In cross-sections of the head the immense eyes lie close to 

 the brain and take up all the space between it and the epi- 

 dermis. Each is innervated by a short nerve passing from the 



