578 EHABDOPLEURID.E. 



a distinct Subclass, we have an account from AUnian and 

 from G. O. Sars : the former has studied our British R. Nor- 

 inani ; whilst the latter has given us an exhaustive memoir 

 on a Scandinavian species, i?. mirabilis, founded on a care- 

 ful examination of living specimens. The most significant 

 peculiarities of Rhabdopleura are the structure of the 

 lophophore and the disposition of the tentacles upon it. 

 Instead of a circular or crescentic stage round which the 

 arms are ranged in a continuous series, we have here two 

 long and narrow and more or less divergent lobes, ex- 

 tending out from the anterior region of the body in a 

 dorsal direction, and each bearing a double row of ten- 

 tacles. The tentacular wreath has disappeared ; and in 

 its place we have two independent tentaculiferous pro- 

 cesses, which are highly flexible and mobile, reminding 

 us more of the " gill-tentacles " of Terebratula than of 

 the lophophore of the ordinary Polyzoa. 



Another striking feature of the present form is the 

 large shield-like organ ("buccal shield ") situated be- 

 tween the two orifices of the alimentary canal, which is 

 probably the homologue of the Molluscan foot"^. Ac- 

 cording to Sars^s observations, it is by means of this 

 organ (in the absence of all protrusor muscles) that the 

 polypide slowly draws itself up to the mouth of the 

 cell. 



* Allman, indeed, takes a diiferent view of this organ, based chiefly on 

 observations made on the early development of the polypide, and is inclined 

 to compare it with the mantle of a Lamellibranehiate moUusk. His account 

 of the evolution of the bud I am unable to follow so as fully to appreciate 

 its bearing on the point in question ; but there seem to me to be serious 

 objections to the proposed interpretation. Sars regards the shield as the 

 equivalent of the epistoiue of the Freshwater Polyzoa ; and Prof. Ray 

 Lankester identifies it with the Molluscan foot, — conclusions in which (with 

 our present knowledge) I fully concur. 



