394 Royal Society i — 



rent varieties, the essential unity of the segmental organ within the 

 indicated limits can be convincingly established. 



Upon this organ, under different circumstances, there devolve one 

 or two or even more functions. Sometimes it is used as a simple 

 discharge tube, conveying externally in a direct manner the fluid of 

 the general cavity of the body. This variety is exemplified in the 

 segmental organs which are distributed, in the genera Lumbricus 

 and Nais, throughout all that part of the body which is situated 

 posterior to the Reproductive band. In this latter region two or 

 more of these organs are so modified as to become the basis whereon 

 are developed the generative structures. 



Here the author enters upon a minute account (illustrated by 

 figures) of the history of this organ in Lumbricus and Nais^ showing 

 the changes of outward form which it undergoes in several species of 

 these genera. 



He points out in this place that the segmental organ, as it occurs 

 in Lumbricus and Nais, is paralleled by the so- called water- vascular 

 system of the Rotifera : as in the former so in the latter, the ciliated 

 tubes communicate freely with the general cavity ; in both, the cur- 

 rent raised by the cilia travels from within outwards ; and he con- 

 tends that the reproductive structures are ingrafted upon, or deve- 

 loped from one, two or more of the ciliated tubes in the Rotifera, as 

 from the segmental organs of Lumbricus and Nais. 



Arenicola and Terebella form a group in which the segmental 

 organ deviates in a remarkable apparent manner from that of Lum- 

 bricus and Nais. It forms a series of elongated sacculi, which are 

 attached to the ventral wall of the general cavity on either side of the 

 median line. Each sacculus, although single at its distal end, is 

 divided at its attached end into two tubular limbs, one of which 

 communicates directly with the exterior, while the other opens imme- 

 diately into the general cavity of the body. Through the latter limb 

 the ova and sperm-cells are introduced into the perivisceral chamber, 

 while in the reverse direction the fiuid of this chamber is discharged 

 externally. The author has never been able to discover how the 

 germ- and sperm-cells (respectively in the female and male) escape 

 out of the general cavity ; but he trusts that he has given a new 

 and satisfactory demonstration of the mode in which they enter that 

 cavity. The genera Arenicola and Terebella comprehend the only 

 Annelids in which the germ-elements in the female, and the sperm- 

 cells in the male, are ushered into, and are required to sojourn for a 

 season in the fluid of the general cavity of the body. 



He indicates in this place that the segmental organ of the Sipun- 

 culidse (amongst the Echinoderms) corresponds both in its structure 

 and relations to that of Arenicola and Terebella, with this difference 

 only, that in the latter a special and peculiar development of the 

 blood-vascular system occurs around and in the vicinity of the seg- 

 mental organ, whereas in the Sipunculidse this system scarcely exists 

 and never receives any enlargement. The segmental organs in the 

 genus Synapta stand in an intermediate position between those of 

 Holothuria and those of Sipunculus. In Synapta one or more 



