82 
THE VOYAGE OF H.M.S. CHALLENGER. 
Hence it appears that the Thecosomata resemble the Bulloidea more than the Gymno- 
somata. 
We must now inquire what are the special affinities of these latter, that is to say, 
what are the Tectibranchia to which they are most nearly related ? 
B. Gymnosomata. 
As in the case of the Thecosomata we have based our inquiry on the most primitive 
of the group, that is, mainly on the Pneumonodermatidse, and especially on Dexio- 
branchsea. We have already shown in the Report on the Gymnosomata 1 that the 
Pneumonodermatidse are the most primitive of the naked Pteropoda, and that Dexio- 
branehsea is the least specialised among them. Wagner 2 is quite wrong in regarding 
Clione as more primitive than Pneumonoderma, and the latter as derived from the former. 
a. In most Tectibranchs there is a proboscis of the acrembolic type, that is, produced 
by the evagination of the anterior part of the oesophagus, like the rather short one of 
Dexiobranchsea and Clione (PI. V. fig. 4, a), the somewhat longer one of Pneumonoderma 3 
and Spongiobranchsea , 4 and the very long one of Clionopsis. 5 Among the Anaspidea 
(Aplysioidea) we find a similar rather short proboscis in Aplysia, Notarclius , 6 &c. 
b. Like the Gymnosomata the Aplysioidea have two pairs of cephalic tentacles 
(Aplysia, Notarchus (Fig. 4, on p. 83), Dolabella, &c.); the anterior pair correspond to 
the labial pair of the Gymnosomata, and the second pair to the nuchal tentacles of these 
latter, for the olfactory nerve terminates in their interior and the optic nerve at their 
base. In the Bulloidea, on the other hand, we know that the cephalic tentacles fuse to 
form the shield which is of so much importance in connection with the burrowing habits 
of these animals. 
c. The fins of the Gymnosomata are comparable to those of the Aplysioidea. 
Yon Jhering 7 refuses to admit the homology of the parapodia of Gastropteron and 
the other Tectibranchia with the “ pteropodia ” of the Pteropoda. If 'these organs are 
absolutely homologous with the epipodia of the Prosobranchs which the French school 
of the Sorbonne (Lacaze Duthiers and his pupils) regard as pallial in nature, that is a 
point which I should not like to affirm ; nevertheless, I regard these latter as also pedal 
in origin. 8 I maintain, however, that the parapodia of the Tectibranchs and the fins of 
the Pteropods are strictly homologous. 
1 Zool. Chall. Exp., part lviii. p. 67. 2 Die Wirbellosen des weissen Meeres, Bd. i. p. 119. 
3 Zool. Chall. Exp., part lviii. p. 6, fig. 1, 4- 4 Ibid., p. 19, fig. 2, 1. 
6 Ibid., pi. iii. fig. 1. 
6 Vayssiere, Recherches zoologiques et anatomiques sur les Mollusques Opistobranches du Golfe de Marseille, 
i. Tectibranches, loc. tit., p. 83. 
7 Vergleichende Anatomie des Nervensystemes und Phylogenie der Mollusken, p. 249. 
8 Paul Pelseneer, Sur la valeur morphologique de Pepipodium des Gastropodes rhipidoglosses, Comptes rendus, t. cv. 
p. 578. 
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