23-2 prop. w. a. haswell ! 



Nepheidia. 



Characteristic of the nephridia] system in JCxogone fustifera is the fact that 

 in both sexes the nephridia of each pair unite completely towards the time 

 of sexual maturity. In immature stages they are represented by pairs of 

 narrow ciliated tube? of the usual character, with ciliated funnels. But in 

 this and subsequent stages the walls of these organs are composed, not of few 

 and comparatively large cells, as observed by Goodrich (14) in other Syllids, 

 but of numerous small cells, as noticed by Claparede (2. p. 213, pi. 13. 

 fig. 2 a) in the mature male of Exogone clavigera. 



In the female in the earliest stage of sexual differentiation observed 

 (PI. 18. fig. 48), each of the segments from the 12th backwards contains a 

 pair of small ova. each enclosed in a sac with small-celled walls having 

 a tubular outgrowth opening at the nephridioporc, The ciliated funnels at 

 this stage appear lo be entirely separate from the sacs. The derivation of the 

 ova is unknown. 



In specimens bearing embryos, in the segments on which embryos are 

 situated, and in several in front and behind, nephridia are present having the 

 following features. Each is a small oval sac with small-celled walls opening 

 on the exterior through the nephridiopore, and of a short narrow canal 

 leading from it to the adjacent nephridial funnel on the other side of the 

 septum. The sac is contractile, the contractions being rhythmical though 

 irregular, and the rhythm corresponding roughly with that of the contrac- 

 tions of the ventral vessel. The contractions have the effect of drawing the 

 walls of the organ towards the attached external end. In sections it is seen 

 that a transverse branch from the ventral blood-vessel ends blindly imme- 

 diately in front of the nephridial sac, and it is probable that it is the 

 contractions of this branch vessel that stimulate the contractions of the wall 

 of the sac. At the staee now referred to, the sac does not contain an ovum 

 — merely an irregular coagulum. But specimens without embryos were 

 found as already stated having the nephridia in almost exactly the condition 

 just described except that each sac contains a small ovum. 



The ovum increases in size, distending the enclosing sac till its wall 

 becomes reduced to a very thin membrane — the tubular efferent part being 

 permanently retained with little alteration. As the two ova in each segment 

 increase in size their enclosing sacs grow inwards towards one another. 

 Eventually they ineet in the middle line (PI. 18. fig. 49) and unite to form a 

 single spacious sac enclosing the two ova and having on each side the 

 original communication with the exterior. When the two sacs communicate, 

 the ti\o ova, instead of lying one on either side of the segment as at first 

 (fig. 50), take up a new position one in front of the other, each extending 

 transversely right across the cavity of the segment (fig. 51). 



When the ova are discharged from the nephridial sacs each at once becomes 

 attached by one end to the small area of the ventral surface (immediately 



