xn 
INTKODUCTION. 
It will be observed that the term “ septal chambers ” is used in 
the text instead of air-chambers,” which is in common use to 
designate the chambers marked off by the septa in the shells of 
some Cephalopods. Eegarding the contents of these chambers 
various opinions have been held. Valenciennes^ argued that con- 
sidering the manner in which the mollusc adheres to its shell, 
advancing as it grows, it is difficult to conceive that the chambers 
were not completely empty, because if there were any gas or liquid 
in them it could only have entered in the form of a secretion 
the discoloured spirit. ... It seems possible that the irritation created by 
these creatures [the parasites] may have been the cause of the Xautilus quitting 
its shell. If so, it tends to show that the animal possesses the power of 
suddenly detaching the shell-muscle from the shell. This is not surprising 
when the feebleness of the scar and the fact that a thin transparent membrane 
is secreted between the muscle and the scar upon the shell are taken into con- 
sideration. The muscle evidently has such a slight hold on the sliell that a 
very small effort of contraction would release it. If the theory be correct that 
the Nautilus moves forward a certain space to form a fresh chamber, relaxing 
its hold on the shell by the lateral muscles of attachment, were tliere not some 
means of holding on there would be considerable risk of the shell falling away 
from its inhabitant. The siphon then, as suggested by Eeeve, may be the 
means by which the shell is held in position during that process. It might be 
stretched the required length, or if movable within the si phonal tube it might 
be pulled forward the length of the compartment, to be parted off by a new 
septum, and still leave sufficient in the old siphonal tube to hold the shell from 
slipping away. When the specimen observed by Mrs. Kenny quitted its shell, 
it appears to have snapped the siphonal membrane, for only about half-an-inch 
is still attached to the body.” The above opinion of Eeeve (which was also 
von Buch’s) is confirmed by MM. Vrolik and Van Breda in their observations 
upon the anatomy of the Nautilus (Ann. Mag. Nat. Hist. vol. xii. 1843, p. 173). 
These authors concluded from their dissection of a specimen “that the animal 
is attached to the shell only by the siphon. The two muscles, by which it 
should adhere to the shell, according to the opinion of many naturalists, are 
applied only to the horny membrane or girdle, which Owen has so well described 
in his remarkable work [Memoir on the Pearly Nautilus, 1832]. This mem- 
brane does not itself adhere to the shell, so that there is no difficulty in 
detaching the horny membrane in question without the slightest laceration, 
both from the surface of the muscles and from that of the shell between which 
it is situated.” 
In connection with this subject attention may be drawn to an interesting 
paper by H. Dewitz (Schriften der physikal.-okonomisch. Gesell. zu Kdnigs- 
berg, 1880, Abth. ii. p. 165, figs. 2, 3), in which the occurrence of three oval 
muscular impressions is described uponxa cast of the body-chamber of Ortho- 
ceras regulare, Schloth. 
1 “ Nouvelles Eecherches sur le Nautile Flamb4 {Nautilus pompilius, Lam.),” 
in Archives du Mus. d’Hist. Nat. 1839, p. 257. 
