542 



resemble very closely tiie intestinal caeca in some species of the family 

 when invested by the developing ova, and the position of the orifices 

 relatively to one another and to the middle line answers very well to 

 the position of the apertures of communication of the caeca with the 

 intestine. Further it has to he observed that, were Williams's 

 account to be accepted as correct, we should be obliged to admit that 

 the segmental organs and sexual glands of Aphrodita and Polynoë are 

 framed on a type totally unlike that observed in any other annelid : 

 he represents the former as complexly branched tubes, not opening 

 into the perivisceral cavity, and the latter as being developed in the 

 interior of the former ; whereas in other Annelides the segmental 

 organs are unbranched. and nearly always open into the perivisceral 

 cavity, and the sexual ylands are developed in the walls of the latter. 

 Moreover I have found in those species of Polynoë whose structure I 

 have specially studied, segmental organs not markedly diiferent from 

 those of other annelides, and sexual glands having the normal rela- 

 tions. It is therefore not inadmissible to suppose that Williams's 

 representation of these structures may have been founded in some way 

 or other on erroneous observations or inferences. It seems to me most 

 probable that what Williams took for the segmental organ was a 

 part of the dorsal branched portion of the intestinal caeca, and that 

 his ciliated efferent duct was the ciliated neck of communication bet- 

 ween the caecum and the intestine. The intimate manner in which 

 the ovaries are related to these caeca would help to account for this. 

 AVhen he states positively that the ova are most clearly seen in the in- 

 terior of the branching tubuli it is evident that he had mistaken the 

 yellow cells for young ova; what he figures as spermatozoa does not at 

 all resemble the spermatozoa of Polynoë^ which are rod-like, but are 

 evidently ciliated epithelial cells. Ke this as it may, it is perfectly 

 conclusive as shewing that Williams's descriptions, whatever be 

 their precise explanation, arc erroneous, that the fully-developed sexual 

 products both in Aphrodita and Polynoë are found floating freely in 

 the perivisceral fluid, which could not be the case, were they formed 

 in the interior of caecal tubes opening only on the exterior. 



While there is some little difficulty in explaining Williams's 

 statements on this subject, Ehlers's descriptions and figures, on the 

 other hand, are so clear as to leave no doubt at all of the nature of the 

 error into which he has fallen. In describing the anatomy of Poly- 

 noë pellucida he states : — »Segmentalorgane habe ich vom zweiten 

 Segmente an in allen völlig ausgebildeten Segmenten gesehen. Ihre 

 Lage haben sie im Hohlräume des Klytrenträgcrs oder lîasalstiickes 

 des Riickciuirrus und ragen von (l;i in den Segmeiitalramn unter 



