541 



imperfectly and the stages in the formation of tlie sexual products not 

 at all.« A careful examination of the subject has led me to the con- 

 clusion that both of these observers were looking at the same thin^ 

 from different points of view, but were entirely in error in regarding 

 it as the segmental organ. 



Williams's description of what he regarded as the segmental 

 organs of Aphrodita is as follows •'. »They appear under the cha- 

 racter of pyriform tubuli , commencing or ending in a single ex- 

 ternal orifice. Internally they are lined by a ciliated epithelium, the 

 cilia being large, dense, and acting with great force and vigour. The 

 current raised by these cilia sets up on one side and down on the 

 other. The ciliary epithelium ceases at the point where the primary 

 branches divide. All the rest of the organ is unciliated and filled with 

 the reproductive products. This part is elaborately branched; — the 

 branches twining round the diverticula of the stomach. *»» The in- 

 dividual tubes are bridled on one side and glandular on the other.« 



»The author thinks it probable that if the roots or attached ends 

 of these organs could only be followed through the integuments to 

 their extreme outlets, they would be found to divide into two limbs, 

 an ingoing and an outgoing ; a fact which would account for the clearly 

 divided ciliary currents as they are seen in the dilated portions of the 

 organs. * ♦ ^^ None of the branches communicate with the general 

 cavity of the body« 4. 



I have not had the opportunity of dissecting any fresh or well-pre- 

 served specimens of Aphrodita\ but the above account is stated by 

 Williams to apply also to the segmental organs of Polyno'ê^ and I 

 find the arrangement in that genus so totally different from that de- 

 scribed by Williams in the memoir above quoted that I have been 

 led to an explanation of his descriptions and figures which at least re- 

 conciles them with what I find to exist in those Polynoidae in which 

 I have worked out this point. 



In the first place it is to be noted that Williams gives no clue 

 to the position of the external orifice ; he admits in fact that he had 

 not been able to follow the canal through the integuments. In the se- 

 cond place in the figure which he gives of the alimentary canal and 

 supposed segmental organs in Aphrodita (1. c. pi. VIII. fig. 26) he 

 either has omitted altogether a portion of each intestinal caecum, or as 

 I incline to believe, has represented it as the segmental organ. Thirdly, 

 the figures which he gives of the segmental organs oî Polyno'é (fig. 27) 



3 Phil. Trans. 1858, p. 134. 

 * 1. c, pp. 134 and 130. 



23** 



