The different authors agree but little on the cephalic appendages of the gymnosomatous Pteropoda, and many of them consider all these appendages as tentacles. In short their homologies are very obscure.3
We have stated that in the Gymnosomata there exists in a constant manner two pairs of tentacles properly so called. I do not think it will be rash to identify these appendages with the two pairs which the 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 tentacles, for example, in Hyalaea, Cleodora, and Creseis, 1 Cuvieria and Spirialis,2 Tiedemannia3 and Cymbulia.4
With several of these animals the tentacles present rudimentary 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 displacement 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, Pneumodermon, and Cirrifer (compare figs. 2, 4, and 7) The "lips" situated between the cones of Clione are not then equivalent to the lips of the other Gymnosomata, in which there is no developed hood like that of Clione. They would be a differentiation of the internal wall of the buccal cantys which is prodnced behind the buccal appendages.
In this manner we see that with Clione, as with Pneumodermon and Cirrifer, the buccal appendages are inserted on the internal wall of the buccal cavity.
At the same time it must be remembered that this front portion of the buccal cavity may be regarded as not part of the true oral cavity, but as only an "introvert" like that of proboscidiferous Gastropods.