RHABDOPLEURA MIRABILIS. 4l 



wall of these cliambers was always of very dark horn-brown 

 colour^ and so little transparent that the axial cord, also here 

 running in the middle, was but dimly discernible through the 

 wall. This dark colour was particularly intense at one of the 

 ends, and appeared to proceed from the axial cord fixed in 

 the middle of the septa, and here somewhat enlarged. Its 

 exterior horny substance seemed to be directly continued 

 into the adjacent chamber of the stem. Allman considers 

 these closed chambers, wherein he has thought to perceive a 

 stratum of large polygonal cells — of which, however, I have 

 not been able to observe the slightest trace in any that I 

 have examined — as statoblasts, and thinks that they are 

 formed by the posterior enlarged part of the contractile cord. 

 This seems to me, however, to be far from probable, at least 

 in reference to the closed chambers examined by me ; for 

 they looked far more like remains of old cells decayed, owing 

 to the destruction of the Polypide, as is found to be the case 

 with other P^lyzoa in the oldest part of the polyzoarium. 



It will be seen from the foregoing description that the 

 genus Rhabdopleura differs in nearly all essential points from 

 the ordinary Polyzoa far more than Allman seems to have 

 conceived. If w^e compare what is here communicated with 

 the chief points which have been briefly enumerated by various 

 authors, for instance, Allman, Hyatt, &c., others, as charac- 

 teristics of the Polyzoa in general, and which, therefore, are 

 considered as essential marks most intimately connected with 

 the idea Polyzoa, the anomaly of this form becomes so strik- 

 ing as finally even to justify a doubt as to whether it really 

 can be referred to the class of Polyzoa. First and foremost 

 stands the want of a so-called endocyst or mantle, which 

 sharply distinguishes this form from all other known Polyzoa, 

 all of which possess such an appendage. This mantle is so 

 essential a component part of a Polyzoon that it is difficult 

 to imagine one without it. One would rather imagine the 

 ectocyst wanting, as this plays a far less important part in the 

 economy of the animal, generally remaining passive, and 

 properly only serving as a protection for the soft animal. 

 The mantle is likewise a characteristic for the Tunicates and 

 the Brachiopods, which two classes have also been united by 

 M. Edwards with the class Polyzoa, under the common ap- 

 pellation of Molluscoidea (Hackers Himatega or Mantle- 

 animals). 



Next, and as a consequence of the absence of a real endo- 

 cyst, the retraction and protrusion of the animal in the Rhab 

 dopleura are effected in a manner totally different from tliat 



