ix PHYLUM ANNULATA 197 



rounded aperture, the pupil, and enclosing a mass of 

 gelatinous matter, the lens. 



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. 103 and 104, neph.\ occurring in all the segments of 

 the body. Each of these has an external opening 1 or nephri- 

 diopore^ fine circular pore capable of being widened or 

 contracted, situated on the ventral surface not far from the 

 base of the ventral cirrus ; and each opens internally into the 

 ccelome through a ciliated bell or funnel, the nepkrostome, 

 projecting through the mesentery into the cavity of the 

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

 lies. 



Nereis is unisexual. The sexual elements, ova or sperms, are 

 formed from temporary masses of cells the ovaries or testes, 

 which are developed towards the breeding season by a pro- 

 liferation of the epithelial cells of the membrane (peritoneum) 

 lining the ccelome and the structures it contains. 



Ova and sperms, when fully ripe, are discharged, and reach 

 the exterior in the case of the sperms probably through the 

 nephridia, in the case of the ova, which are much too large 

 to pass out in this way, probably through apertures tempor- 

 arily formed by rupture of the body-wall ; and impregnation 

 takes place by contact between the two sets of elements 

 while floating freely in the sea-water. 



Other Annelids which may be found along with Nereis 

 on the sea-shore will be found to resemble it in the seg- 

 mented character of the body and the presence of para- 

 podia with setee, but to differ from it in the general shape, 

 the number of the segments, the form of the parapodia, 

 the arrangement and shape of the setae, the form of the 



