582 PEOF a. J. ALLMAN ON THE 



zoon will help us in arriving at a conclusion regarding these 

 points. 



The more important features by which Rhahdopleura differs 

 from the typical marine Polyzoa will be found (1) in its cres- 

 centic lophophore ; (2) in the tentacular series not being con- 

 tinued from the bases of the arms round the body of the lopho- 

 phore ; (3) in the lateral rather than terminal position of the 

 mouth ; (4) in the possession of a remarkable shield-like organ 

 which is attached to the body of the lophophore between the 

 mouth and anus ; (5) in the possession of a chitinous rod which 

 extends through the tubular coencecium, and gives attachment to 

 one end of a contractile fleshy cord, which at the other end is 

 fixed to the body of the polypide ; (6) in the absence of an endo- 

 cyst lining the cavity of the zooecium, and in the further absence 

 of a tentacular sheath. 



A somewhat closer comparison of Rhahdopleura with an ordi- 

 nary polyzoon will bring out some points of great interest, and 

 will, I think, suggest the true significance of its singularly ab- 

 errant features. One of its most anomalous characters is the 

 apparent absence of an endocyst and tentacular sheath. In the 

 spirit specimens I failed to find any thing but what seemed the ob- 

 scurest indications of them ; and on these I would lay no stress, 

 for the careful investigations of Sars on the living animal render 

 it highly probable that neither endocyst nor tentacular sheath as 

 usually understood are to be found. I believe, however, that 

 Rliahdopleura is not without their homologues. These may be 

 partly recognized in the fleshy contractile cord. This cord 

 may be regarded as an endocyst which has become separated 

 from the chitinous ectocyst, while the approximation of its 

 walls has brought about the almost complete obliteration of its 

 cavity. Where, however, the cord is attached to the polypide, 

 it spreads in the form of a membrane over the whole of the 

 alimentary canal. This membranous extension of the cord re- 

 presents the anterior part of the endocyst with the tentacular 

 sheath ; and Sars has already suggested the possibility of the 

 endocyst being here found. Posteriorly a still greater transfor- 

 mation has taken place ; for in all the older parts of the coence- 

 cium we find the continuation of the cord in the condition of a 

 chitinous rod, which, however, still presents in its lumen a trace 

 of the original eudocystal cavity. The endocyst in receding from 

 the ectocyst carries with it the longitudinal muscular fibres which 



