Still more than Boas, it is Pelseneer who has largely contributed to the knowledge of these 

 Mollusca. In 1885 he expressed his opinion ^) that the Pteropoda were related to the Euthyneura 

 and this conviction was kept by him in the years following "). In the Challenger Reports 

 we find his most extensive paper, written on this subject ''). In this work, after treating in details 

 the confused nomenclature and establishing the precisely different types (something which can 

 only be mentioned here in a few lines), he discussed briefly and distinctly the different opinions 

 with respect to the systematic place of the Pteropoda. 



It would be going too far to expatiate long, on what Pelseneer has explained in his 

 work with an abundance of proofs, taken from anatomy. Some of the results of his study, 

 though somewhat abridged, follow here "'). 



A. The Pteropoda do not constitute among the Mollusca a class of the same value as the 



Cephalojjoda, Gastropoda, etc. 



B. The Pteropoda are not primitive Mollusca, but are a derived and recent group. 



C. Thev have no affinity with the Cephalopoda. 



D. They are Gastropoda, in which the adaptation to pelagic life has so modified their external 



character, as to give them an apparent symmetry. 



E. Among the Gastropoda they do not constitute a distinct class, not even an order. 



F. They belong to the Euthyneura and among these to the Tectibranchiate Opisthobranchs. 



G. The Pteropoda are polyphyletic in their origin ; the Thecosomata and Gymnosomata are 



two independent groups. 

 H. The Thecosomata have descended from the Bulloidea. 

 /. The Gjmnosomata have descended from the Aplysioidea. 



From these results we learn that the group of the Pteropoda, according to Pelseneer, 

 is an unnatural union of Thecosomata and Gymnosomata, two sections which must be classified 

 separately to the Tectibranchia, and so the Thecosomata to the Bulloidea (Cephalaspidea) and 

 the Gymnosomata to the Aplysioidea (Anaspidea). 



In this short survey on the subject, there can be no question of going into details with respect 

 to Pelseneer's arguments, and it is sufficiënt to draw the attention to the fact that the Pteropoda 

 are of polyphyletic origin. Pelseneer denies, that the fins of the Thecosomata and Gymnosomata 

 should not be homologous ; according to him the fins of both the groups are parapodia. 



In the fifteen years that have passed since Pelseneer's work appeared. his opinion and 

 that of Boas on the Gastropoda-nature of the Pteropoda, has graduallv found more adherents. 

 Though few researches have been made since 1888 and only some articles on a single specimen 

 or a single organ have been written, yet different zoologists have become convinced of the 

 correctness of Pelseneer's assertion. 



i) The cephalic appendages of the Gymnosomatous Pteropoda and especially of Clione. Quavterly journal Mier. Sc. (2) 

 Vol. 25, p. 506. 



2) Recherches sur Ie système nerveux des Ptéropodes. .\rch. de Biol. T. VII, p. 127. Description d'un nouveau genre de 

 Ptéropode Gymnosome. Buil. se. Dép. du Nord. p. 226. 



3) Zool. Chall. Rep. LVIII, Gymnosomata, 1S87. LXV, 1888 (Thecosomata). Prt. LXVI (Anatomy of Pteropoda). 



4) Challenger Report. LXVI, p. 95, 96. 



