BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 315 



occupying the centre of the general cavity posteriorly. This description, no 

 doubt, refers to a Borlasian ; but he states that the same arrangement occurs in 

 the Ommatopleans, and represents in Polia* a series of transverse fibres as 

 forming a platform (plancher) at the anterior and upper portion of the general 

 cavity of the body, indicating its presence in his figures by a dark shading. No 

 such arrangement of transverse fibres has been seen by me, but the characteristic 

 ciliated oesophagus occupies this situation, and has probably misled the observer. 

 The somewhat erroneous views he entertained with respect to the relations of the 

 corpusculated fluid of the proboscidian chamber may be seen by a glance at one 

 of his figures,! which depicts in Polia sanguiriibra the proboscidian bodies as 

 floating in what he calls the genital cavity, and in which the genital caeca are 

 supposed to lie. I cannot corroborate his statement that these discs become 

 much more numerous at the epoch of reproductive activity. The diminished size 

 of the chamber may cause a slight crowding anteriorly, but this is not an increase. 

 He did not recognise the complete muscular sheath for the proboscis and the 

 proboscidian fluid. Dr Johnston likewise confounded the cavity-proper of the 

 proboscis with the general cavity of the body ; and Dr Williams, % who styled the 

 canal the oesophageal intestine, stated that it opened externally on the side of the 

 body, not far from the head, after the manner of the Sipunculidse. M. Van 

 Beneden,§ however, alludes to the sheath for the proboscis in Polia obscui'a, and 

 compares the fluid and discs therein to pale blood. Professor Keferstein,|| 

 again, follows the majority of his predecessors, in so far as he also describes the 

 proboscidian discs as floating in the general cavity of the body, in which, more- 

 over, he locates the proboscis (Riissel) ; thus ignoring the special and complete 

 muscular sheath just described. 



The structure of the proboscidian discs, and the highly organised condition of 

 the transparent liquid in which they float, point them out as being, in all proba- 

 bility, concerned in nutrition, as first mentioned by M. de Quatrefages, though 

 he likewise associated generation therewith. Some very interesting questions, how- 

 ever, are raised by their entire absence in the curious Polia involuta, Van Ben., 

 especially to those who, like the late Dr Williams, consider the fluid analogous to 

 the peritoneal or perivisceral fluid in the true Annelids — a fluid, we may remark, 

 which Professor Huxley^" considers as the true blood, while he thinks the red 

 fluid in the branching vessels analogous to the water vascular system in the 

 Annuloida. If in Polia involuta the proboscidian fluid had been more important 

 in nutrition than that in the vessels, it certainly would not have given way to 

 the latter. It is to be remembered, too, that this absence coincides with the 



* Op. cit. fig. 1, pi. xviii., and fig. 1, pi. xix. | Op. cit. pi. xxii. fig. 1. 



+ Report, Brit. Assoc. 1851. § Op. cit. p. 26. 



|| Zeitsch. fiir wiss. Zool. xii. pp. 69 and 71. 



% Notes of Lectures at the Roy. Coll. Surgeons, Med. Times and Oaz., March 7, 1868. 



