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,,0n the other hand, the highly developed digestive system, the presence of an anus, the 

 juxtaposition of the mouth and anus, and finally the bilateral Lophophore, are all characters pe- 

 culiar to the Polyzoa, and entirely foreign to the Hydrozoa, 



,.It is clear that we have under observation in the Rhabdopleura, a form of animal life 

 which stands as it were in the middle between the Hydrozoa and Polyzoa, or forms a tran- 

 sition from one to the other; one of those ,,perplexing forms" which will not fit rightly any- 

 where in the systems of zoologists. 



,,When finally, and as the object of the whole investigation, we will give account of 

 our conception of the Rhabdopleura, 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 Rhabdopleura is undoubtedly, like many other animals which at present inhabit 

 the greater depths of the sea, and with some of which we have in the latter times become 

 acquainted, a ccnj 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 authors agree in referring to the main type or trunk (phylon) 

 of the Molluscs are usually supposed, among the other animal types, to shew the greatest 

 affinity with the Vcrmes; and they are even considered by many zoologists as not being mol- 

 luscs at all, but as genuine worms. Their affinity to worms has however not been demon- 

 strated by any evidence, and distinct transition-forms between the two are not known. The 

 Rhabdopleura shews now evidently that the Polyzoa are not most closely related to the worm- 

 type, but to the type of the coelent crates, and especially to the class of Hydrosoa. The Po- 

 lyzoa have already in the earliest primordial times (for fossil remains of them are found in 

 the lowest Silurian formations) developed themselves 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 consist in the following points. In stead of the simple abdo- 

 minal cavity of the Hydrozoa, with a single aperture which functions as both mouth and anus, 

 there is formed an intestinal canal with special walls, dividing itself into 3 sections: gullet, 

 stomach and intestine, 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 Rhabdopleura, but no more. The following phases of this development 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 (invagination) and attached round 

 about the basis of the tentacular Corona, (formation of the tentacular sheath, vagina) whereby 

 the animal that formerly was free in its cell now becomes attached to the anterior part of 

 the cell, and the << Duplicated system of special retractor and protractor muscles, all these muta- 

 tions have not yet taken place in the Rhabdopleura. This animal has thus remained statio- 

 nary in the first stage of development from a Hydrozoon to a Polyzoon, but must nevertheless 



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