x PHYLUM ANNULATA 461 



narrow epidermal cells, most abundant on the prostomium and 

 peris tomium, have probably to do with this faculty. 



Tho organs of excretion the segmented organs or nephridia 

 (Fig. 362) are similar to those of Nereis, but somewhat more 

 complicated. They are slender tubes which occur in pairs in all 

 the segments of the body except the first three and the last. 

 Externally each nephridium opens by one of the small nephridio- 

 pores which have already been mentioned as occurring on the 

 ventral surface ; internally it ends in a funnel-shaped ciliated 

 extremity with a crescentic slit-like aperture, the nephrostome 

 (nst), opening into the cavity of the segment in front of that 

 in which the external aperture occurs. The tube is thrown into 

 three loops attached to the posterior surface of the corresponding 

 septum by a fold of membrane. Two parts are clearly recognis- 

 able^^J inner narrow and an outer wide part : in the former the 

 naijU^entral lumen is a perforation through the axis of a string 

 of cells, and is thus intracellular : it is lined in parts with cilia 

 arranged in two rows; in the latter (the terminal vesicle) the 

 passage is lined by cells, and is thus intercellular, and there is a 

 thick muscular investment. The nephridia are abundantly 

 supplied with blood by means of nephridial branches of the 

 ventral vessel. 



Reproductive Organs. The Earthworm is hermaphrodite. 

 There are two pairs of very small flattened testes (Figs. 360, 363, te, 

 te), partly divided into a number of digitate lobes, situated in the 

 tenth and eleventh segments. A pair of comparatively large sacs, 

 the anterior vesiculce seminales (ant. ves. sem) lie partly in the 

 cavity of the ninth segment, but extend into the tenth, where 

 they coalesce in the middle to form a large median sac of some- 

 what irregular form, the anterior spemn-rcservoir (ant. sp. res). 

 The anterior pair of testes project into this, and the cells destined 

 to form the sperms, developed in the former, pass by dehiscence 

 into the large median cavity. On either side is a large ciliated 

 funnel, or rosette (fun), leading outwards from the interior of the 

 reservoir. A second pair of vesjculse seminales (mid. ves. sem), 

 situated in the eleventh segment, also open into the anterior 

 sperm-reservoir. A third pair (post. ves. sem), situated in the 

 twelfth segment, unite in front to form the posterior sperm-reservoir 

 (post. sp. res), which lies in the middle of the cavity of the 

 eleventh segment. The posterior pairs of testes have the same 

 relation to this as the anterior pair have to the anterior reservoir ; 

 and a posterior pair of ciliated funnels (fun) lead outwards from 

 its cavity. Each ciliated funnel passes into a narrow, somewhat 

 convoluted duct, the vasefferens,&ud the two vasa efferentia of each 

 side unite to form a vas deferens or spermiduct (v. def), right or 

 left as the case may be, which passes almost straight backwards to 

 open by the corresponding male aperture on the fifteenth segment. 



