RHABDOPLEURA MIRABILIS 43 



forms" which will not fit rightly anywhere in the system of 

 zoologists. 



When, finally^ and as the object of the whole investiga- 

 tion, we will give account of our conception of the Rhabdo- 

 pleura, and decide on the class to which we will refer it, our 

 opinion is that these questions, like so many others, can only 

 be properly answered through the medium of the Darwinian 

 theory. 



The Kliabdopleura is, undoubtedly, like many other ani- 

 mals which at present inhabit the greater depths of the sea, 

 and with some of which we have in the latter times become 

 acquainted, a very old form, which in its organisation has 

 still retained several features from the time when the animal 

 type that we call Polyzoa first developed itself from a lower 

 type. 



The Polyzoa, which most anthers agree in referring to the 

 main type or trunk (phylon) of the Molluscs are usually 

 supposed among the other animal types, to show the greatest 

 affinity with the Vermes ; and they are even considered by 

 many zoologists as not being molluscs at all, but as genuine 

 worms. Their affinity to worms has, however, not been 

 demonstrated by any evident and distinct transition- form 

 between the two are not known. The Rhabdopleura shows 

 how evidently that the Polyzoa are not most closely related 

 to the worm-iy-^e, but in the type of the Coelenterates, and 

 especially to the class ofHydrozoa. The Polyzoa have already 

 in the earliest primordial times (for fossil remains of them 

 are found in the lowest silurian formations) developed them- 

 selves from the Hydrozoa by transmutation. We have in 

 the Rhabdopleura manifestly such a form of Polyzoa in course 

 of development out of a form of Hydrozoa. The changes 

 which must take place in order that a Hydrozoon can be 

 transmuted into a Polyzoon consists in the following points. 

 Instead of the simple abdominal cavity of the Hydrozoa, 

 with a single aperture Avliich functions as both mouth and 

 anus, there is formed an intestinal canal ivith special walls, 

 dividing itself into three sections — gullet, stomach, and intes- 

 tine, which last ascends alongside of the stomach and gullet, 

 terminating with an exterior aperture or anus in the vicinity 

 of the mouth. This formation is completed in the Rhabdo- 

 pleura, but no more. The following phases of this develop- 

 ment which consist in the formation of a wide sack-like 

 contractile endocyst or mantle, which in its anterior part is 

 detached from the ectocyst or cell, involved in itself (inva- 

 gination) and attached round about the basis of the tentacular 

 sheath, vaginal, whereby the animal that formerly was free in 



