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XII. — On the Homology of the Organs of tlieTunicata andthe Polyzoa* ByGEOEGE 

 James Axlman, M. D., F. R. C. S. I., M. R. I. A., Professor of Botany in tlie 

 University of Dublin. 



Read January 26, 1852. 



X HOUGH the close affinity between the Tunicata and the Pohjzoa has been 

 generally acknowledged, yet the full extent to which the organization of the one 

 is represented by that of the other does not appear to have been hitherto re- 

 cognised by the zoologist. I propose in the present communication to point out 

 some apparently imnoticed instances of homological identity, while I sliall en- 

 deavour to show that almost every modification of form in the organization of 

 the one is, by the easiest transition, convertible into a corresponding form in 

 the other ; that they are both, therefore, constructed on precisely the same type, 

 and must constitute one and the same great natural group. 



In order to render this subject intelligible, it will be necessary in the first 

 place to fix the terms indicative of the various aspects of the Tunicata and the 

 Polyzoa, terms which are so vaguely used by different aiithors as to give rise 

 to great confusion in description. In the determination of the anterior and 

 posterior aspects, there would seem to be no difficulty, as the former must ma- 

 nifestly be assumed as that to which the mouth is directed, while the posterior 

 will then of course be the aspect directly opposed to this. The determination 

 of the dorsal, or superior^ and of the ventral, or inferior aspects, is not so easy. 

 I believe, however, that the cephalic ganglion, or its homologue, must be here 

 our true guide, and that its position will always correspond with the dorsal, or 

 superior aspect of the animal, to which the ventral will then consequently be 

 diametrically opposed. Mr. Huxley, in his admirable memoir on Salpa and 



* Polyzoa Thompson, synonymous with Bryozoa Eheenbehg. Thompson's name has priority 

 of date over that of Ehrenberg, and should, therefore, injustice to its founder, and in obedience 

 to the laws of Natural History nomenclature, be adopted. 



VOL. xxn. 2 



