GENERATIVE SYSTEM OF NAIS. 243 



as well as from the setting of the ciliary currents at the mouth and base 

 of the tube, it seems beyond the possibility of a doubt that these organs, 

 in this ordinary state, subserve the function of discharging externally 

 the fluid contained in the general cavity of the body ; nevertheless, from 

 their small diameter, it would appear that they are not capable of con- 

 veying externally either the normal corpuscles of the chylaqueous fluid 

 or the spermatic products. The preceding description applies only to 

 the ' segmental organs ' distributed throughout that part of the body of 

 the worm which is situated behind the reproductive mass. 



(634.) It now remains to investigate the claims of this organ to a new 

 and hitherto unthought-of relation with the generative or reproductive 

 structures ; for in this worm two, or possibly more, of these segmental 

 organs on either side of the body undergo a remarkable increase of size 

 and variation of form. 



(635.) In Nais it often happens that only one on either side is thus 

 evolved. At some seasons, however, especially when the weather has 

 continued for some time warm and dry, individuals may be found in 

 which two on each side have undergone a marked development ; gene- 

 rally, however, two segmental organs only, the one being developed 

 into the male, the other into the female moiety of the reproductive 

 system, are necessary to the generative maturity of the individual. 



(636.) The two segmental organs which form the basis of the repro- 

 ductive masses are observed, even on the first view, to be very similar 

 in general outline to those which are repeated in every ring of the body, 

 with which indeed they are identical, differing only in size and in the 

 shape of some of their parts. The dilated portions correspond with the 

 equivalent parts of the other organs ; and the umbrella-like extremities, 

 which are the same in form on the ovarian and testicular sides, are the 

 counterpart of the free ciliated ends of the ordinary organs. The inter- 

 mediate ciliated tubes are specially distinguished only by their greater 

 length and thickness. It is a difficult point to settle the mode in which 

 the generative masses are related to the ciliated tubes ; it may, however, 

 be constantly seen that the ovarian mass moves to and fro with the 

 dilated portion of the tube. It is impossible to detect the opening by 

 which the ova arrive in the interior of the utriculus of the tube ; that 

 they do get there, however, is certain. From this portion of the tube 

 they escape by the external orifice. 



(637.) The testes on the other side bear the same relation to the tube 

 as the ovary. The utriculus upon one side is represented by the ejacu- 

 latory pouch on the other. At present it is beyond the power of science 

 to explain why these organs, in only one or two annuli of the body, 

 should be implicated in the sexual development, while all the rest 

 remain in abeyance in the condition of mere ' excretionary tubes'; or 

 why the ciliated tube of one side should be changed into the female 

 system, and that of the other into the male. 



