78 
THE VOYAGE OF H.M.S. CHALLENGER. 
consider each group separately, taking its own special characters into account, and if in 
this way we inquire with which of the Tectibranchs each group has the greatest affinity, 
we shall arrive at the conclusion that the two groups are not so closely related to each 
other as they are to the particular forms of Tectibranchs for which they have each the 
closest affinity. 
This is an impression which must have been produced upon every zoologist who has 
examined, even in a cursory manner, the organisation of these animals, for the two 
groups exhibit such clearly marked differences, and each forms such a homogeneous 
whole, that it is quite impossible to derive one of them from the other, or to find for 
them an immediate common ancestor. 
It is only by limiting oneself to the study of a single form (as Wagner has done in 
the case of Clione, 1 and attempting thence to construct the phylogenetic history of the 
Pteropoda, that one can regard the Thecosomata as the ancestors of the Gymnosomata. 2 It 
is true that by following this method one arrives at the strange result that the Pteropoda 
have been derived from the Heteropoda, and have given origin to the Cephalopoda. 3 
Boas was the first to formulate the opinion of the separate origin of the Thecosomata 
and Gymnosomata, and to assert that the two groups are “ independent of each other.” 4 
There is no need to recapitulate here the distinctions between the two divisions ; 
they have been sufficiently expounded in the Report on the Gymnosomata, 5 and in the 
Summaries on the Thecosomata and on the Gymnosomata (pp. 37 and 55). But I must 
dwell for a few moments on the statement made by Boas, 6 “ that the fins are not 
homologous in the two groups.” This is an opinion which I do not share. The fins, 
both of the Gymnosomata and Thecosomata, are the modified lateral margins of the foot, 
and the differences which they present are almost the same as those which exist between 
the Bulloidea and the Aplysioidea. 
In the Bulloidea the pedal surface is continuous with the natatory lobes, e.g., Acera 
and Gastropteron), and there is no special creeping surface. In the Aplysioidea, on the 
other hand, these natatory lobes are distinct from the rather narrow creeping surface, 
which is clearly marked off (e.g., Aplysia, Notarclius, Oxynoe, &c.). 
The Gymnosomata also present an arrangement analogous to that of the Aplysioidea, 
but carried to an extreme ; the natatory lobes are quite separated from the portion of the 
foot corresponding to the creeping surface. 
Embryology shows further that these organs (the fins) are homologous in the 
Gymnosomata and Thecosomata. Fol 7 has shown that the fins of the Pteropoda cor- 
1 Die Wirbellosen des weissen Meeres, Bd. i. p. 119. 
2 Von Jhering, on the other hand, regards the Thecosomata as the descendants of the Gymnosomata (Vergleichende 
Anatomie des Nervensystemes, &c., p. 273), whilst Grohhen holds that the Limacinidse (Thecosomata) are the most 
primitive Pteropoda (Morphologische Studien, &c., Arb. Zool. Inst. Wien, Bd. v. p. 240). 
3 Wagner, Die Wirbellosen des weissen Meeres, Bd. i. p. 22. 4 Spolia atlantica, &c., loc. cit., p. 179. 
6 Zool. Chall. Exp., part lviii. pp. 4-6. 6 Spolia atlantica, &c., loc. cit., p. 179. 
7 Sur le developpement des Pteropodes, Archives de Zool. Expdr., ser. 1, t. iv. p. 193. 
