ex INTRODUCTION. 



be regarded as the homologue of the foot (Woodcut, 

 fig. xlv., I, F). The organ, indeed, has changed its cha- 

 racter and function in correspondence with the change in 

 the conditions of life; but it maintains its original rela- 

 tions to the other elements of structure, and is easily recog- 

 nizable in its disguise. In Rhabdopleura (which is a true 

 Polyzoon) it seems to resume to some extent its primitive 

 office, in the absence of a muscular system, and at the 

 same time exhibits a considerable increase in size. We 

 are not surprised at finding but slight traces of the organ 

 in forms which have taken on a stationary habit of life, 

 nor at its total disappearance in many of them. But 

 the presence of this most distinctive Molluscan structure 

 within the Polyzoan group is a striking indication of its 

 afifiuities. 



Secondly, as to the gills of the Mollusca, they cor- 

 respond in all essential particulars with the tentacular 

 corona of the polyzoon. If we compare the latter, as it 

 appears amongst the Phylactolcemata or in Rhabdopleura 

 (Woodcut^ fig. xlv. 1, 2), with the cirriferous appendage 

 of the Brachiopod, we find the closest resemblance be- 

 tween them"^, and not a mere resemblance, but such an 

 identity of relation to other portions of structure as 

 clearly indicates their homology. The foot amongst the 

 Brachiopoda is very slightly developed ; but the disposi- 

 tion of this organ, of the arms, and of the oral and anal 

 openings is the same in them as we find in Rhabdopleura. 

 Turning to the other Acephala, the arrangement and 

 mutual relation of these parts (as Ray Lankester has in- 

 sisted) are for the most part identical in the Lamellibranchs 



* The rao))ility of the tentacular plumes of Rhabdopleura is a point in 

 which they agree witli the iirms of the Brachiopod. 



