Section IV, 1918 [95] Trans. R.S.C. 



Branchioderma and Branchiotrema 

 By Professor A. Willey, D.Sc, F.R.S.C. 



(Read May Meeting, 1918). 



With respect to sucii classification as we are here concerned with, 



it might be more proper to speak of a classification 



than of the classification of the animal 



kingdom. — T. H. Huxley. 



In this paper an attempt is made to use the respiratory organs 

 as the basis of a binary system of zoological classification. It is of 

 the essence of the theory of evolution that descent by heredity is 

 associated with ascent in organization, just as the cell-lineage of the 

 embryo leads to cell-differentiation in the adult. The many instances 

 of partial and complete degeneration do not invalidate this general 

 issue, inasmuch as the extreme of degeneration implies the extreme of 

 adaptation to a conditional existence. Hence a synopsis of classifica- 

 tion should exhibit both of these aspects in different ways, namely, as 

 lines of descent and as grades of organization. The history of zoology 

 is, in great measure, the history of classification and of its underlying 

 principles, so that if it is possible to contribute towards clarit^^ of 

 expression on this subject, no apology is needed for the eflfort. 



In 1873 Sir Ray Lankester proposed the terms Diploblastica for 

 coelenterate animals, and Triploblastica for bilateral animals. For 

 certain reasons they were, subsequently, replaced by Enterocoela 

 and Cœlomocœla respectively, these being the two grades into which 

 Lankester (1900) divides the branch Enterozoa. An attempt to 

 establish the binary cleavage of Triploblastica has been made by 

 Professor Hatschek (1911). In this author's phyletic table, two 

 branches of bilateral animals are distinguished by their modes of 

 formation of the third germinal layer or mesoblast, namely, Ectero- 

 coelia and Enterocoelia. The latter, which are not to be confused 

 with Lankester's Entercoœla, are further subdivided into Dipleuridia 

 and Chordonia; they include Chaetognatha, Echinoderma, Tunicata 

 and Vertebrata. Some discrepancy is unavoidable owing in part to 

 the circumstance that at no period has physiological adaptation kept 

 even pace with morphological differentiation. It remains to be seen 

 whether alternative methods may not possess sufficient merit to 

 warrant separate treatment. 



By bringing out in classification the contrast between the two 

 leading modes of obtaining oxygen, through the skin and its dériva- 



