GYMNOSOMATA de Blainville. 



1824. Gymnosomata dc Blainville, Diet. d. Sci. Nat., vol. XXXII, p. 271. 



1885. Deiitocephala Wagner, Die Wirbellosen des weissen Meeres, Bd. I, p. 119. 



1886. Pterota Boas, Spolia atlantica, K. dansk. Vidensk. Selsk. Skriv., 6 Raekke, Bd. IV, p. 14. 



The Gymnosomata form a group among the Tectibranchia which exhibits the nearest 

 affinity- with the Aplysioidea. For that reason they are not so clo.sely related to the Thecosomata 

 (which are Bulloidea modified in adaptation to their pelagic life) as to unite these two groups 

 to the order "Pteropoda". In spite of many points of resemblance, generally based on 

 convergence, the whole organisation of the G)mnosomata is quite different from that of the 

 Thecosomata. Both these groups pos.sess a different origin. 



This important fact, already alluded to by Boas ^), has been amply discussed by 

 Pelseneer ") who, correcting some of Boas' suggestions, was the fir.st to trace out the phylogenetic 

 affinity of the Gymnosomata (see also p. 6). 



I-'rom a systematic point of view the Gymnosomata were very unsatisfactorily known, 

 not yet twenty years ago. Now, it is true, that the study of these animals, still more than that 

 of the Thecosomata, is made difficult by several facts. First the animals are generally very 

 small, and their bodies being .strongly and irregularly contracted in the preserving fluid, it is 

 not cas\' to describe their true shape. In the second place the buccal appendages, on which the 

 systcmatization is mainly based (at least as regards the species of the same genus), are nearly 

 always retracted in preserved specimens, and not visible externally. Finally, the gills are often 

 concealed under the large folds of the body-wall. And so, every point for a right conception, 

 regarding the systematic position of any member of the Gjmnosomata, can only be obtained 

 by means of careful examination and dissection. 



Notwithstanding the acknowledgment of the difficulties above mentioned, one can scarcely 

 supjjress an angry feeling against such authors as Ouoy and Gaimard, Bruguiere, and even Rang. 

 For these naturalists, describing "new species" without indicating any specific distinctness, and, 

 indeed, destitute of any knowledge about the organisation of these animals, have been the 



1) Spolia atlantica, p. 179. 



2) Chall. Rep. LXVI, p. 82—88. 



