30 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



Station 



84 



87 



88 



90 



92 



Date 



1S /7 



"h 



is/ 7 



45° 26' 



25° 45' 



21 /7 



53/ 7 _24/ 7 



N 

 Position w " 



48° 4' 

 32° 25' 



46° 48' 



27° 46' 



46° 58' 

 19° 6' 



48° 29' 



13° 55' 



Depth in m. 













0— 50 



— 



— 



— 



— 



— 



50— 100 



— 







— 



— 



8 



100— 250 



— 



— 



— 



— 



— 



250— 500 



— 



— 



1 



1 



— 



500— 750 



1 



1 



— 



— 



— 



750—1000 







— 



— 



— 



1000—1250 



— 





— 



— 



— 



1250—1500 



— 



— 



— 



— 



— 



A difference from the other species of Cymbulia is 

 seen in the absence of a ventral lobe on the fin. 



Shell unknown. 



Dimensions: The largest individuals measure 

 about 15 mm. across the fins, the length of the body 

 being about 3 mm. 



Geographical distribution. In contradistinc- 

 tion to Cymbulia peronii this species seems to inhabit 

 the colder waters of the Northern Atlantic, having been 

 taken only between latitude 45° 26' and 48° 29' N. 



Number of individuals of Cymbulia borealis n. sp. 



General remarks on thecosomatous pteropods. 



The "Michael Sars" brought home 27 species of the- 

 cosomatous pteropods, 11 of which belong to the family 

 Limacinidae, 14 to theCavoliniidae and 2 to theCymbuliidae. 



Leaving out of account the two species Procymbulia 

 michaelsarsii and Cymbulia borealis, both new to science 

 but still imperfectly known, the material includes practi- 

 cally all the species of thecosomatous pteropods previously 

 known from the North Atlantic, some of them in quan- 

 tities which have enabled us to make statistical investiga- 

 tions of different kinds, others under conditions which 

 have allowed of our entering upon the solution of 

 questions bearing upon the general relationship between 

 the various groups. 



The systematical results have already been 

 referred to (see Limacina balea, retroversa and trochi- 

 formis, the varieties of Clio pyramidata, Diacria trispi- 

 nosa, and Cavolinia inflexa). 



In this place I propose to give an account of some 

 general results bearing upon comparative anatomy and 

 geographical distribution. 



1. Remarks on comparative anatomy. 



As already mentioned (p. 20) the prevalent view of 

 modern investigators with regard to the relationship between 

 the different groups of thecosomatous pteropods is princip- 

 ally based upon the hypothesis of Boas (1886) regarding 

 the phylogenetic development of the Cavoliniidae from 

 the Limacinidae through a rotation (of 180°) of their body 

 relative to the head. 



This hypothesis rests upon the fact that important 

 organs for organ-systems) of the body are found in 

 opposite positions within each of the two families: the 

 dorsal pallial cavity of Limacinidae is found on the ventral 

 side in Cavoliniidae, the anus in the Limacinidae is found 

 on the right side, but on the left side in the Cavoliniidae, 



and so on, while all the organs of the head, tentacles, 

 mouth, radula etc., correspond in both families. — One 

 organsystem, the genital system, which with its various 

 parts belongs to both head and body, fully bore out the 

 hypothesis of rotation, the genital gland, with the proximal 

 part of the duct being found in opposite positions in the 

 two families, while the distal part of the duct with the penis 

 is always found at the same place beside the right tentacle. 

 While therefore the genital duct of the Limacinidae runs 

 directly from the genitial gland to the opening along the 

 right side of the body, it takes its orgin in Cavoliniidae 

 from the gland on the left side and then sweeps round the 

 ventral side of the neck until it finally reaches the opening. 



The hypothesis of Boas was illustrated by three dia- 

 grammatic figures (reproduced in textfig. 26), which seemed 

 so clear and convincing, at the same time harmonizing 

 so well with the anatomy of the species investigated, that 

 any further proof of its correctness might seem unne- 

 cessary. A similar hypothesis was independently set forth 

 by Pelseneer (1888), and the rotation theory has been 

 generally adopted by later authors. 



The whole proces of rotation must according to this 

 theory have taken place in times past, as the most primi- 

 tive form among the Cavoliniidae was considered to be 

 the one (Creseis) in which the position of the visceral 

 organs is directly opposite to that in the typical Limacinidae. 



The existence of archaic thecosomatous pteropods 

 living at the present day has however lately been main- 

 tained by Pelseneer (1906), who observed the right-sided 

 position of the pallial cavity and the presence of a ctenidium 

 in Peraclis triacaniha. And in the "Michael Sars" material 

 two species occur, which as will be shown lead from this 

 archaic genus Peraclis on the one hand to the two diverging 

 groups of the Limacinidae and the Cavoliniidae on the other. 



