506 
PAUL PELSENEEB. 
do not think it will be rash to identify these appendages with 
the two pairs which tbe Gastropoda Euthyneura (Opistho- 
branchia and Pulmonata) possess, and which occupy the same 
position among these animals as with the Gymnosomata. 
In the Thecosomata we find a pair of rudimentary ten- 
tacles, for example, in Hvalaea, Cleodora, and Creseis , 1 
Cuvieria and Spirialis , 2 Tiedemannia 3 and Cymbulia . 4 
With several of these animals the tentacles present rudi- 
mentary eyes, Creseis for example . 5 
If they do not present eyes in the adult state they possess 
them in some stage of the development, as in Tiedemannia 
and Spirialis . 6 
This pair of tentacles is, in my opinion, equivalent to the 
oculiferous nuchal pair of the Gymnosomata. As to the 
anterior pair, its disappearance is explained by the displace- 
ment of the swimming lobes, which encircle the head and 
between which the mouth opens. The development of the fins 
in this position has caused the disappearance of the anterior 
tentacles. 
Besides the two pairs of tentacles properly so called, we 
have seen that most of the Gymnsomata possess buccal 
appendages; such are Clione, Pneumodermon, Cirrifer. 
Without prejudging anything as to the morphological value 
of these appendages, I believe that they have the same origin, 
however varied their aspect may be in the three genera above 
named. 
Apparently it seems that with Clione they are inserted 
around the mouth, while with Pneumodermon and Cirrifer 
they are inserted on the internal wall of the buccal cavity. 
But it should be noted that in Clione there exists a hood which 
can fall back. This hood covers the buccal cones, and its 
opening corresponds to the buccal opening of Clionopsis, 
1 Gegenbaur, ‘ Untersuchungen uber Pteropoden,’ p. 8. 
2 Souleyet, loc. cit., vol. ii, pp. 199 and 209, pi. xii, fig. 32, and pi. xi, fig. 15. 
3 Gegenbaur, loc. cit., p. 60. 
4 Gegenbaur, loc. cit., p. 45. 
6 Gegenbaur, loc. cit., p. 8, Taf. ii, fig. 1. 
« Krohn, ‘ Beitrage zur Entwickelungsgeschichte der Pteropoden/ p. 21. 
