REPORT ON THE PTEROPODA. 
59 
Grobben 1 also affirms that the Pteropoda ought to be included in the class Gastro- 
poda, but without deciding to which group they are related. 
In 1885 I pointed out, when treating of the cephalic appendages, that the affinities 
of the Pteropoda are with the Euthyneura (Pulmonata and Opisthobranchia), 2 and since 
then I have defended their precise affinities with the Opisthobranchia, 3 and especially 
with the Tectibranchia. 4 
Lastly, Boas, in the morphological introduction to a work systematic in the main, 5 
has followed out rigorously the view of de Blainville, and it is to be regretted that this 
part was not more extensive, and that he did not give at full length a demonstration 
of the affinities of the Pteropoda with the Tectibranchiate Opisthobranchs, and of the 
genealogical relations of the two groups. 
There are no other instances of avowed adhesion to this view. The “ class ” Pteropoda 
still keeps its position everywhere ; and its so-called affinities with the Cephalopoda 
are maintained by the powerful support of timid souls, who not being able to make up 
their minds to modify the Cuvierian system, and having really no opinion of their own, 
retain the generally received ideas as a matter of prudence. 
From this rapid historical sketch, it appears that there are three different theories 
regarding the systematic position of the Pteropoda. 
1. They form a distinct class. 
2. They are Cephalopods. 
3. They are Gastropods. 
We must therefore attempt to answer the following questions : — 
1. Are the Pteropods Cephalopods ? 
2. Are the Pteropods Gastropods ? 
If we obtain a negative answer to these two questions, then we must clearly retain 
the Pteropods as a distinct class, but if either of them be answered in the affirmative, the 
“ class ” Pteropoda must be abandoned. 
In order to answer the two questions we shall compare the Pteropods successively 
with the Cephalopoda and the Gastropoda, on the basis of those anatomical characters 
which are common to the Thecosomata and Gymnosomata. 
1 Morph ologische Studien iiber den Ham- und Geschlechtsapparat sowie die Leibeshohle der Cephalopoden, Arb. 
Zool. Inst. Wien, Bd. v. p. 245. 
2 On the Cephalic Appendages of the Gymnosomatous Pteropoda, Quart. Journ. Micr. Sci., vol. xxv., N. S., p. 506. 
3 Recherches sur le systeme nerveux des Pt^ropodes, Arch, de Biol., t. vii. p. 127. 
4 Description d’un nouveau genre de Pteropode Gymnosome, Bull. Sci. Dip. Nord, 1886, p. 226. 
6 Spolia atlantica, &c., K. dansk. Vidensk. Selsk. Skriv., 6 Raekke, Bd. iv. p. 12. 
