PHYLOGENETIC RELATIONS OF THE GENERA OF 

 GYMNOSOMATOUS PTEROPODA. 



In the anatomical part of this Report, I shall show the close affinities that these 

 Mollusca possess to the Tectibranchiate Opisthobranchs. 



The zoologists who have carefully studied the organisation of the Thecosomata and 

 Gymnosomata have always recognised the great differences that separate the two groups ; 

 and many have thought that they have not had a common origin. Boas, however, 

 was the first who clearly stated this opinion, 1 against which no solid argument can be 

 opposed. 



Here I have only to consider the origin of the Gymnosomata. According to my 

 opinion, their origin ought to be found in the group Aplysiidae, Opisthobranchia which 

 already show a remarkable adaptation to swimming habits. In the anatomical part of 

 this Report I shall show the great and numerous resemblances which exist between the 

 organisation of the Gymnosomata and that of the Aplysiidae. 



With regard to the relations of the different genera of Gymnosomatous Pteropoda 

 among themselves, we find rather primitive genera which have retained much of their 

 resemblance to the primitive stock, and others which are highly specialised. Between 

 these two extremes, there are a whole series of forms, by means of which the evolution 

 of the group could almost be traced, and which are not difficult to arrange in a phylo- 

 genetic tree, according to their affinities. 



The stage of evolution of the group, which each genus represents, is chiefly 

 characterised by the development of the respiratory organs. Thus, Dexiobranchsea, 

 which among all the known genera is the most primitive, only possesses the lateral 

 gill corresponding to that of the Aplysiidae. Spongiobranchsea has, moreover, a very 

 simple posterior gill, a specialisation of the posterior ciliated ring which remains very 

 late in the foregoing genus. Pneumonoderma shows a great increase in the complexity 

 of this posterior gill, by the formation of four crests radiating from the posterior 

 quadrangular ring, which corresponds to the membranous ring of Spongiobranchaia. 



Clionopsis already shows a retrogression, the lateral gill having quite disappeared ; and 



1 Spolia atlantica, p. 14. 



