CONTRIBUTION TO KNOWLEDGE OF RHABDOPLEURA. 641 



no difficulty in referring the lophophoral arms and tentacles 

 (as well as those of Phoronis and the Eu-polyzoa) to the same 

 primary organ — the postoral portion of the architroch or 

 ciliated band — from which the molluscan gill-plume is derived. 

 Whilst the buccal-discs of Rhabdopleura and Cephalodiscus 

 may possibly, after all, be shown by embryological fact to be 

 ventral and not dorsal organs, it is well to remember that on 

 the assumption that they are dorsal, we are not bound to con- 

 clude that they are necessarily PROstomial, and correspond to 

 the velar area of the trochosphere. On the contrary, they may 

 represent (as may the epistome of Phoronis) a region nearer the 

 anus, the prostomium being suppressed, as it is in 

 Lamellibranch molluscs. Thus, then, Allman^s sugges- 

 tion, that the huccal-shield of Rhabdopleura represents the 

 mantle area of Lamellibranchia, may prove to be correct. 



The buccal- shield of Rhabdopleura, and still more so that of 

 Cephalodiscus, is so peculiar that it is impossible to feel satis- 

 fied with any identification of it which does not rest on 

 embryological grounds. 



The fact that the buccal-shield secretes the tubarium, that in 

 the bud the shield is very large relatively, and is more or less 

 reflected over the small lophophor-rudiments and small poly- 

 pide body, are in favour of identifying this organ with the 

 Lamellibranch's mantle. Such an identification appears to 

 involve the assumption that Rhabdopleura and Cephalodiscus 

 are degraded Lamellibranchs. 



All possibilities must be kept in view, and it is only when 

 we have the embryology of Rhabdopleura that these questions 

 can be decided. Even then it may possibly turn out that 

 Rhabdopleura has a perverse kind of larva, which will defy the 

 attempt to distinguish its trochospheral surfaces. 



Questions for Futuee Observers of Rhabdopleura. 



I will venture here to set down some of the points in regard 

 to Rhabdopleura, concerning which further information seems 

 especially desirable, and likely to be obtainable, if definitely 



