GYMNOSOMATA. 



More than 150 specimens of gymnosomatous pteropods 

 were procured during the "Michael Sars" expedition, of 

 which about 100 belong to the well-known species Clione 

 limacina. 



Besides the arctic form one other, Pneumodertnopsis 

 macrochira Meisenheimer, occurs in considerable number 

 (24) but otherwise the material consists of single or a very 

 few specimens of no less than seven different species, 

 probably all new to science. 



I must confess that the investigation of these few 

 specimens has given me more trouble than the whole 

 group of thecosomatous pteropods put together, and I 

 fully agree with Tesch (1904) in his remarks with regard 

 to the difficulties to be overcome in the examination of 

 the material and in consulting the literature. The more 

 or less contracted bodies of the animals seldom exceed 

 5 to 8 mm. in length, with the systematically important 

 organs of the buccal mass, as a rule, in the centre. Conse- 

 quently these organs are in many cases not described, or 

 even mentioned, while new species are founded upon the 

 varying external appearance of the animals. 



The papers of Boas (1886) and Pelseneer (1888) 

 represent a great advance in our knowledge in so far as 

 they have based their systems upon special organs, the 

 form and development of which seemed characteristic 

 enough for systematic grouping. The buccal appendages 

 and the gills having proved of service in both systems, 

 their results agree very well and their systems still form 

 the bases of our classification of the gymnosomatous 

 pteropods. 



During my attempts, however, to determine the position 

 of the new forms contained in my material within the old 

 systems I was led to doubt the value of the latter as 

 expressions of the real natural relationship between the 

 species. Thus more than once it happened, that species 

 with practically identical buccal organs should according 

 to these systems be referred to different families because 

 their posterior gills were differently developed, while in 

 a natural^ system the buccal organs and especially the 

 radula must, as original mollusc-organs, be considered as 



having a systematic value far above that of the posterior 

 gill, a secondary organ of the highly specialized group 

 of gymnosomatous pteropods. 



I therefore undertook the work of investigating the 

 buccal apparatus of all the species at my disposal, even 

 if they were represented by only a single specimen. I 

 soon found that even in much contracted individuals the 

 whole proboscis can be separated by dissection without 

 any essential damage to the appearance of the animal. 

 After having studied and drawn the entire buccal mass, 

 I prepared the radula for a further study by boiling it in 

 caustic potash. 



In table IV (p. 50—51), the results of this investi- 

 gation are combined with the information found in the 

 literature of the subject, so that the systematic value of 

 different organ-systems may here be directly compared 

 with each other. 



All the species of gymnosomatous pteropods, the 

 radulae of which have been investigated, are in this table 

 arranged according to the similarity of their median tooth. 

 At the first glance these teeth are seen to represent very 

 different types, which occur practically unchanged within 

 the large groups; thus all the forms belonging to the 

 family Pneumodermatidae have one type of median tooth, 

 while another type is characteristic of the Notobran- 

 chaeidae. In both families a reduction of the median 

 tooth is seen to take place. 



Not quite so uniform are the median teeth of forms 

 which have been included within the two families Clionop- 

 sidae and Clionidae, and future investigations may prove 

 that these families do not represent really natural groups. 

 The median tooth of the Clionopsidae is of a type similar 

 to that of the Pneumodermatidae, while that of the Clio- 

 nidae is more like the type of the Notobranchaeidae, but 

 other organs, as for example the jaw, prove that they 

 are distinct. 



Other characters of the radula, such as the number 

 and shape of the lateral teeth, are not so useful systemati- 

 cally as the median tooth. The number of lateral teeth 

 within ea c h transverse row varies somewhat, probably with 



