Chap. XIII.] GONADS OF WORMS. 49! 



becomes greatly enlarged, and two pairs of large sacs 

 (the already mentioned reservoirs), are developed, 

 which completely obscure, and, indeed, have been 

 very frequently mistaken for the true testes. The 

 only explanation which has been given of the pre- 

 sence of nephridial canals and efferent ducts in 

 the same segment is that of Lankester, who has sug- 

 gested that each segment was typically provided with 

 two pairs of segmental organs, the superior of which 

 ordinarily become aborted. Some worms (Eudrilus), 

 present indications of the presence of both sets of 

 organs, and in others (Ocnerodrilus), the ordinary 

 nephridia are not developed in the segments which 

 carry the generative ducts. 



While both the leech and the earthworm have but 

 a single pair of ovaries, the former is provided with a 

 single median vagina, in place of a duct opening 

 directly to the exterior ; in the leech, moreover, the 

 ova, when set free from the ovary, are not taken up by 

 the open mouths of the oviducts, and as the wall of 

 the oviduct is directly continuous with the investment 

 of the ovary, they do not, as in most vertebrates, for 

 example, pass first into the body cavity ; they make 

 their way directly to the exterior. 



As may be supposed from their radial symmetry 

 in adult life, the Echinodermata have their genera- 

 tive organs radially disposed ; in the more primitive 

 forms, such as Brisinga, the glands are arranged by 

 pairs, and extend along the whole length of each arm, 

 while the generative pore is placed in the proximal 

 region of each ray ; as centralisation increases, the 

 glands become less elongated, and the pores are placed 

 within the area of the disc ; in the Ophiuroids the 

 glands are completely confined to the disc, where they 

 form five racemose groups ; in the Echinoids the in- 

 terradially-placed pairs become fused, and only five 

 sets of genital glands are to be made out. In the 



