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THE VOYAGE OF H.M.S. CHALLENGER. 
E. Among the Gastropoda they do not constitute a distinct subclass, nor even an 
order. 
F. They belong to the Euthyneura, and among these to the Tectibranchiate Opistho- 
branchs. They differ less from the Tectibranchs than these differ from the other 
Opisthobranchs. The different families which make up the Pteropoda must be 
distributed among the families of the Tectibranchia according to their special affinities. 
G. The Pteropoda are polyphyletic in their origin ; in other words, the Thecosomata 
and Gymnosomata are two independent groups : they have not a common origin and 
they differ more from each other than each one differs from a group of Tectibranchs 
to which it is most closely allied. 
H. The Thecosomata have descended from the Bulloidea. 
I. The Gymnosomata have descended from the Aplysioidea. 
VIII. GENERAL CONCLUSIONS. 
I have shown that the Pteropoda differ less from the Euthyneurous Gastropoda of 
the Tectibranchiate Opisthobranch group than these differ from the other Opistho- 
branchs, and that the different forms which have hitherto been united in one “class” 
Pteropoda find their natural places besides certain families of Tectibranchs. Further- 
more, I have shown that the “ Pteropoda ” as a whole differ less from those Tectibranchs 
wdiich are known as Cephalaspidea and Anaspidea (or Bulloidea and Aplysioidea re- 
spectively) than these together differ from the third group of Tectibranchs, Notaspidea 
(or Pleurobranchoidea). 
In conclusion then I may say : — The Heteropoda were formerly regarded as a distinct 
class like the Pteropoda. But for a long time now they have been placed among the 
Gastropoda, the affinities having been recognised which they bear to the Streptoneura (or 
Prosobranchia), of which, to use the expression of Spengel, they are forms “ modified 
by adaptation to a pelagic mode of life.” 1 
I am strongly in favour of this mode of classifying the Heteropoda according to 
their natural affinities. Yet the Heteropoda are Gastropoda much more specialised in 
their organisation than are the Pteropoda ; and they exhibit differences from all the 
Streptoneura much greater than those which separate the Pteropoda from the Euthy- 
neurous Gastropoda of the Tectibranchiate Opisthobranch group. 
Henceforth, therefore, we should abstain from making a distinct class of the 
Pteropoda, but we should rather distribute the animals which have been called by this 
name among other groups, according to their natural affinities. 
The table on the following page shows the manner in which I propose to classify the 
Pteropoda. 
1 Die Geruchsorgane und das Nervensystem der Mollusken, Zeitschr.f. wiss. Zool., Bd. xxxv. p. 343. 
