REPORT ON THE PTEROPODA. 
63 
life the complete asymmetry is very distinct ; and if in the adult we have an apparent 
external symmetry, there is a real internal asymmetry. And, on the contrary, in the 
Cephalopoda the complete primordial symmetry never disappears for a moment from 
the youngest embryonic stage to the perfect adult state. 
It is not, however, only in the above facts (anatomical and embryological) that we fail 
to find traces of affinities between the two groups. Even in the three points already 
mentioned, as indicating resemblances between them, we shall show that the likenesses 
are not real- but merely superficial. 
1 and 2. The flexure of the intestine and the position of the pallial cavity certainly 
constitute one of the most important and most often quoted arguments in favour of 
the relation between the Pteropoda and Cephalopoda. 1 It is asserted that the flexure is 
“neural” in the C'avoliniidse and in the Cymbuliidse, and that the anus opens in them, 
as in the Cephalopoda, into a ventral pallial cavity. 
The form of the argument is perfectly fair, but yet the conclusion is entirely false, 
because of the inaccuracy of the premises. The flexure of the intestines and the position 
of the pallial cavity in the straight Thecosomata, though apparently similar to those of 
the Cephalopoda, are really due to quite a different process, and that which is primitive 
in the Cephalopoda is secondary in the Pteropoda, as Grobben 2 perceived, so that the two 
are not strictly comparable. 
The fact is, that a truly primitive neural flexure of the intestine and a primitive 
ventral pallial cavity only exist in the three classes — Cephalopoda, Scaphopoda, and 
Pelecypoda. This flexure is brought about by a displacement (considerable in the 
Cephalopoda and Scaphopoda) of the posterior part of the body — a displacement resulting, 
in its turn, from a partial rotation in the neural direction about a transverse axis. 
As regards a haemal flexure, it may be said with truth not to exist. In the Gastropods, 
where the pallial cavity is dorsal, the flexure of the intestine is always lateral , in conse- 
quence of a movement of rotation (quite different from that observed in the Cephalopoda) 
of the posterior part about a short dorso-pedal axis, which has been especially studied by 
Spengel. 3 Further, the terminal branch of the digestive tract may end either above the 
oesophagus (as in many Gastropods), which gives the appearance of a haemal flexure, or on 
the same level with it (as in a good many Opisthobranchs), or even below the oesophagus, 
which would bring about almost a neural, but still always lateral, flexure. 
As regards the Pteropoda, they have the same lateral flexure of the intestine as the 
Gastropods, the anus opening on the right, below the oesophagus in the Gymnosomata, in 
consequence of the reduction of the pedal surface, and about on the same level with it, or 
1 Huxley, On the Morphology of the Cephalous Mollusca, Phil. Trans., 1853, p. 44 ; Gegenbaur, Grundriss der 
vergleichenden Anatomie, p. 378, fig. 190, 1878. 
2 Morphologische studien, &e., Arb. Zool. Inst. Wien, Bd. v. p. 5141. 
3 Die Geruchsorgane und das Nervensystem der Mollusken, Zeitschr.f. wiss. Zool., Bd. xxxv. 
