124 



praeoral cavity, as shewn in 1 Iatscukk's tig. 145, suggest that the ventral cavity may currespunJ 

 with that part of the anterior body-cavity of Ccplialodiscus which hes in the proboscis-stalk, 

 ventrally to the notochord ; and that in fact the proboscis in Ainphioxus is represented only 

 by a part of the proboscis-stalk. It would be interesting to know more of the nature of van 

 Wijue's dorsal praeoral cavity ("Schnauzenhohle"); and it is conceivable that it might be the 

 homologue of the pericardium of the I'.nteropneusta. 



The only group with regard to which it appears necessary to say more is that of the 

 Polyzoa. A large number of writers have assumed — and it appears to me on inade(|uate 

 grounds — that Phoronis is related to tin- Polvzoa. If /'/loronis be so related, it is not foreign 

 to the oljjcct oi this memoir to consider the questic^n in its bearings on Cephalodiscus. 



It may be remembered at the outset that on the occasion of the first discovery of 

 RJiabdopleura by Sars and Norman, no doubt was entertained of the correctness of the view that 

 this animal was an aberrant Polyzoon. The later discovery of Ceplialodiscus^ and its association 

 with RJiabdopleura, naturall\- led to the same view being adopted with regard to the subject 

 ot this Report. With the discovery that Cephalodiscus has affinities in the direction of Balano- 

 glossus, it appeared reasonable to remove it from the I'olyzoa, since these animals had not been 

 shewn to ]jossess any i»f the characters which specially connect Ceplialodiscus with Balanoglossus. 

 What holds good tor Cephalodiscus mu.st, I think, also ai)|il\- to Rhabdopletira. 



In again taking up the study of Cephalodiscus I have seen no sufficient reason for 

 modifying the views I formerly expre.ssed (87) with regard to this cpiestion. It might indeed be 

 maintained that the embryo of C. gracilis with its large internal volk-ma.ss, its conspicuous 

 ventral invagination of ectoderm and its "pyritorm organ" was directly comparable with the 

 larva ot one ot the marine P^ctoproctous Polyzoa. To this might perhaps be added the fact 

 (if it be a fact) that in both groups the alimentary canal of the bud is developed entirely from 

 an invagination of the outer layer. 



It remains to be seen whether the later developmental history of Cephalodiscus will lhr(jw 

 any further light on this particular <]uestion ; but for my own part I do not anticipate that it 

 will tend to strengthen the view that the Pterobranchia are allied to the Polyzoa. In order to 

 accept this view it is necessary either to regard the Ectoprocta as the most primitive of the 

 Polyzoa, or to take the view — which is adopted by Korschelt and Heider in their well 

 known text-book of Embryology — that the Ectoprocta have no close affinity to the Entoprocta. 

 It appears to me that this view is untenable, and I may fortify my own opinion by (juoting 

 that of Prouho '), the excellence of whose work in both subdivisions entitles him to speak with 

 authority, that " Aujourd'hui, aucun des zoologistes iiui etudient les Bryozoaires ne met en doubte 

 "le bien fonde des vues de Xitsche" [to the effect that the Entoprocta and the Ectoprocta are 

 two groups belonging to the same phylum]. The Entojirocta, on this view, furnish many clues 

 to the understanding of the Ectoprocta. In particular, the comjjarative study of the development, 

 taking into account such (juestions as the structure of the larvae of Entoprocta, the metamorphosis 

 of Pcdicellina, and the structure of Cyphonautcs appears to me to shew that the Polyzoa, as 



l) H. rKciiiio, "Contribution a I'llistoiie des Bryozoaires", .Xrch. Zonl. Exp. et den. (2) .K, 1892, p. 641. 



