REPORT ON THE PTEROPODA. 
75 
of Pteropocls and Cephalopods. We might find numerous instances of very dissimilar 
animals, in which a homologous organ is modified in an analogous manner without pro- 
posing to unite them on that account, if the sum total of their organisation showed 
them to be distinct. In this way we ought to deal with the Cephalopoda and Pteropoda. 
On the other hand, a natural classification based upon a comparative examination of 
the whole organisation of the two groups must show, as we have demonstrated in the 
preceding pages, that there is no direct relation between the Pteropoda and Cephalopoda, 
and that they have nothing in common except inasmuch as they belong to the same 
Molluscan phylum. 
The high position which has been accorded to the Pteropoda arises rather from their 
external form than from their structure, as has already been pointed out by Garner. 1 
The adaptation to pelagic life has brought about in these animals a symmetrical exterior 2 
in order to insure the perfection of natation. But this symmetry has proceeded no 
further ; and what clearly separates the two groups is the complete asymmetry of the 
organisation of the Pteropoda as opposed to the perfect symmetry of the Cephalopoda. 
III. ARE THE PTEROPODA GASTROPODA ? 
In the Pteropoda as in the asymmetrical Gastropoda — 
1. The jaws are paired and lateral. 
2. The flexure of the intestine is lateral, what has been improperly called dorsal or 
haemal in the Gastropoda, for the intestine does not bend dorsally in a sagittal median 
plane in the same way as it curves ventrally in the Cephalopoda. 
3. The heart is lateral and has only one auricle ; the kidney is unpaired and lateral. 
4. The unpaired genital gland has only one asymmetrical unpaired genital duct. 
5. The nervous system is asymmetrical as regards the ganglionic masses of the visceral 
commissure and the nerves which spring from it ; the osphradium (olfactory organ of 
Spengel) is unpaired and lateral. 
6. A consideration of the development of the Pteropoda shows that as in the 
asymmetrical Gastropoda the pallial cavity of the Thecosomata is formed to the right 
of the anus, 3 and that the Pteropoda like the Gastropoda are asymmetrical even in the 
larval condition. 4 The first stages of the embryo show the primitive symmetry of all 
Mollusca ; this is soon followed by asymmetry, and in the adult animal, though only as 
regards the external form, there is an adaptive return to the former symmetry, necessi- 
tated by pelagic habits. 
1 Malacological Notes, Ann. and Mag. Nat. Hist., ser. 4, vol. xix. p. 373. 
2 Grobben (Morphologische Stuclien, &c., Arb. Zool. Inst. Wien , Bd. v. p. 240) also interprets the symmetry of the 
Pteropoda in the same way. 
3 Fol, Sur le ddreloppement des Pteropodes, Arch. d. Zool. Expdr., ser. 1, t. iv. p. 198. 
4 Fol, ibid., p. 197. 
