i 9 6 



MANUAL OF ZOOLOGY 



(Fig. in) consists of a darkly pigmented cup, the retina 

 (>-,?), with a small rounded aperture, the pupil, and enclosing 

 a mass of gelatinous matter, the lens (I). 



Fig. hi. — Section through an eye of Nereis, co, cornea; cu, cuticle; /, lens; ?-, 

 layer of rods; re, retina. (After Andrews ) 



The organs which are supposed to perform the function 

 of excretion are a series of metamerically arranged pairs of 

 internally ciliated tubes, the segmental organs or nephridia 

 (Figs. 109 and no, ncph) occurring in all the segments of 

 the body. Each of these has an external opening or nephri- 

 diopore, a fine circular pore capable of being widened or 

 contracted, situated on the ventral surface not far from the 

 base of the ventral cirrus, and opening internally into the 

 ccelom through a ciliated bell or funnel, the nephrostome, 

 projecting through the mesentery into the cavity of the seg- 

 ment next in front of that in which the body of the organ 

 lies. 



