CEPHALIC APPENDAGES OF GYMNOSOMATOUS PTEROPODA. 505 
of the disc adheres, augment the vacuum under this disc, and 
consequently the adherence. 
Except this point of detail, I quite agree with Niemiec on 
the physiological mechanism of the suckers of Pneumo- 
dermon. I am specially persuaded that the prismatic mus- 
cular cells fulfil an important part during the first moments of 
fixation. These cells are especially worthy of the attention 
of those who are interested in the comparative study of dif- 
ferent forms of muscular tissue. 
Cirrifer. 
In 1879, G. Pfeifer described by this name a gymnosomatous 
Pteropod, which much resembles Pneumodermon, whose 
caudal and lateral gills it possesses. 1 
This Pteropod bears two buccal appendages like Pneumo- 
derma, but instead of being provided with suckers these two 
appendages are terminated by two small branches bent round 
in the form of a sickle. According to the drawing of Pfeffer, 
these appendages differ further from those of Pneumodermon, 
in that they are not inserted separately on the buccal wall, but 
reunite in a common stem before arriving at that wall. 
Pfeffer describes two superior or labial tentacles ; it is very 
probable that the nuchal tentacles, very small and retracted, 
have escaped him, as those of Pneumodermon have escaped 
Gegenbaur. 
Summary. 
The different authors agree but little on the cephalic appen- 
dages of the gymnosomatous Pteropoda, and many of them 
consider all these appendages as tentacles. In short their 
homologies are very obscure. 2 
We have stated that in the Gymnosomata there exists in a 
constant manner two pairs of tentacles properly so called. I 
1 ‘ Monatsberichte der Akad. der Wissensch.,’ Berlin, 1879, p. 249, fig. 20. 
2 “ The connections between these conformations and the tentacles of the 
Gastropoda are not yet very clear,” Gegenbaur, ‘ Grundzuge der Vergleichen- 
den Anatomie,’ French translation, p. 481. 
