2 
THE VOYAGE OF H.M.S. CHALLENGER. 
but from that of the entire group, each genus, and usually several species of each genus, 
having been studied. 
As the conclusion of these researches an analytical exposition of the whole organisa- 
tion of each genus might be expected, but this would have extended the ' present Report 
beyond reasonable dimensions, and would have occasioned much repetition, since several 
of the genera of Pteropods are very closely related, and the number of distinct types is 
far from large. Besides, as I have already remarked, the limited time at my disposal did 
not permit me to enter upon an anatomical monograph, and indeed, even if a longer 
period had been available, the following considerations would have deterred me from 
such a course : — 
1. We already possess a very considerable acquaintance with numerous points in the 
organisation of the Pteropoda, thanks to the general works, based upon several different 
genera, of van Beneden, 1 Souleyet, 2 and Gegenbaur, 3 and to the special memoirs, treating 
only of a single form, of Eschricht 4 and Wagner. 5 In many respects an anatomical 
monograph would simply be a reprint of what has been published by these authors. 
2. The systematic position of the Pteropods is the subject of much discussion, and 
their phylogenetic relationships have been very variously interpreted. 
For several years 6 I have followed Spengel, 7 along with Grobben 8 and Boas, 9 in the 
opinion, not shared by most zoologists, that the Pteropoda do not constitute a distinct 
class among the Mollusca, comparable with the Cephalopoda, Gastropoda, Scaphopoda, 
and Pelecypoda. Further, I am, like Boas, of opinion that even within the Gastropoda 
they do not constitute a primary division, but only a group of much lower rank among 
the Opisthobranchiate division of the Tectibranchiate Euthyneura. 
This opinion, it must be added, is not new ; it was expressed during the first half of 
this century by de Blainville. 10 Souleyet 11 was the first, and indeed the only, inves- 
tigator who has attempted to give any proof of it, and he was not very successful, 
1 Exercices Zootomiques, Mem. Acad. Sci. Bruxelles , t. xii., 1839 ; M 4 moire sur la Liinacina arctica, op. cit., t. xiv., 
1841. 
2 Voyage de la Bonite, Zoologie, t. ii. pp. 37-288, 1852. 
3 Untersuchungen iiber Pteropoden und Heteropoden, 1855. 
4 Anatomische untersuchungen iiber die Clione borealis, 1838 ; in Danish, Anatomiske Undersdgelser over Clione 
borealis, K. danslc. Vidensh. SelsJc. Afhandl., 7 Deel, p. 327. 
6 Die Wirbellosen des weissen Meeres, Bd. i. pp. 89-120, 1885. 
8 Die Geruchsorgane und das Nervensystem der Mollusken, Zeitschr.f. wiss. Zool., Bd. xxxv. p. 373, 1880. 
7 The cephalic appendages of the Gymnosomatous Pteropoda, Quart. Journ. Micr. Sci., N.S., vol. xxv. p. 506, 
1885. Description d’un Nouveau genre de Pteropode Gymnosome, Bull. Sci. Dep. Nord, p. 226, 1886. Recherclies sur 
le Systeme Nerveux des Pteropodes, Arch, de Biol., t. vii. p. 127, 1886. 
8 Morphologische Studien iiber den Harn- und Geschlechtsapparaten sowie die Leibeshohle der Cephalopoden, 
Arb. Zool. Inst. Wien, Bd. v. p. 245, 1884. 
9 Spolia atlantica, Bidrag til Pteropodernes Morfologi og Systematik, &c., K. danslc. Vidensh Selslc. Slcriv., 
Raekke 6, Bd. iv. p. 12. 
10 Manuel de Malacologie, p. 480. 
11 Voyage de la Bonite, Zoologie, t. ii. 
