REPORT ON THE PTEROPODA. 7 



swimming. On this subject Souleyet x contradicts the statements of Peron, 2 Eang, 3 and 

 d'Orbigny. 4 It appears, however, that they generally swim in a nearly vertical position 

 with the head uppermost, or else slightly sloping, so that the ventral (pedal) side is 

 turned upwards. They swim by moving the ends of the fins successively backwards and 

 forwards. 



The Gymnosomata, like the other Pteropoda, are nocturnal in their habits, ascending 

 to the surface during the night and sinking to a lower level in the daytime, being driven 

 down by the brightness of the light ; and thus one can scarcely gather Pteropoda on the 

 surface during the day. Mr Murray, however, informs me that he has frequently taken 

 Gymnosomata at the surface in the Arctic seas during the daytime with a hand net. 



It is impossible to distinguish, as d'Orbigny supposed, 5 twilight from nocturnal 

 species. That writer thought that each species inhabits a definite zone of 

 depth, since the different species come to the surface at different times, but it has 

 been experimentally shown by Alexander Agassiz, 6 that no species of the pelagic- 

 fauna descends to more than 100 fathoms. It is then in this bathymetrical zone 

 that the Pteropoda perform their daily oscillations, accompanied by some other pelagic- 

 animals, several of which serve as their prey. 



The anatomical structure of the Gymnosomata will be described in the anatomical 

 part of this Report. 



4. The Gymnosomata of the Challenger Expedition. 



The number of Gymnosomatous Pteropoda collected by the Challenger Expedition is 

 comparatively not very extensive. This can be readdy explained by the fact that these 

 animals are not very numerous in species, and since they are pelagic and nocturnal 

 in habit, they can only be captured by the tow-net during the night, when trawlings were 

 not frequently made, or by sinking the nets to slight depths during the daytime, and 

 also because many common surface forms were not always preserved. 7 



The number of the Challenger Gymnosomata is nevertheless greater than that of 

 any other scientific expedition. The thirteen 8 stations at which they were captured 

 yielded one new genus and specimens of all the other genera, except Clione, furnish - 



1 Voyage de la Bonite, Zoologie, t. ii. p. 273. 



2 Histoire de la famille des Mollusques Pteropodes, Ann. Mas. Hist. Nat. Paris, t. xv. p. 69. 



3 Description d'un genre nouveau de la elasse des Pteropodes, Ann. d. Sci. Nat., ser. 1, t. v. pp. 286, 287. 



4 Voyage dans l'Amerique muridionale, t. v. p. 128. 

 6 Loc. cit., p. 67. 



6 Bull. Mus. Com-p. Zo'61., vol. vi. p. 153. 



7 Narr. Chall. Exp., vol. i. p. 270, note by Dr. John Murray. 



8 I do not include in this number the station (not specified) between Cape Verde and Bahia, at which the Pneu- 

 monoderma (violaceum, I think), recorded in the Narrative of the Cruise, vol. i. p. 219, was taken. This station is 

 unknown to me, because I did not find, in the collection sent to me, specimens of Pnevmonoderma from any station 

 between these two localities. 



