py 
of the Animal of the Nautiloidea and Ammonoidea. 5337 
It has been suggested that the formation of successive 
septa in the shells of these animals is correlated with the 
recurrence of reproductive periods; but Dr. Wiley * states 
definitely that this is not the case in Nautilus, since propaga- 
tion, according to his observations, only takes place after the 
last septum has been formed (p. 747). 
Nautilus is an expert and a rapid swimmer, owing to the 
bouyancy given to the relatively large external shell by its 
series of air-chambers. These chambers are not individually 
air-tight, since they are perforated by the siphuncle, the walls 
of which are permeable +, but collectively they are rendered 
an air-tight and water-tight hydrostatic apparatus, owing 
to the fact that the animal itself completely closes up the 
entrance to the chambers in virtue of its adherence to 
the shell by the muscles and annulus (girdle of Owen). 
The method of formation of the septa, Dr. Willey writes 
(p. 749), “is simple so far as the septum itself is concerned. 
What is not so easy to understand is the manner in which 
the animal glides bodily forwards in its shell, so as to leave a 
space behind it which is destined to become the new air- 
chamber... .(p.750). Keferstein (1865) and Appellof (1893) 
supposed that the mechanism of the forward movement of 
the muscles in the shell consisted of a resorption of muscular 
substance of the hinder border, coincident with a formation 
of fresh muscular substance in front. But this pretended 
resorption of muscle-fibres could not be confined to the ends 
of the muscles, where they abut upon the shell, but must 
affect the entire body of the muscles. There is no evidence 
whatever that anything of the kind takes place, since the 
muscles increase in size pari passu with the growth of the 
animal, and the presence of the concentric muscle-lines on 
_ the shell, visible as they are from the septal suture to the 
anterior border of the muscle-scar on each side, is clearly 
indicative of a very gradual forward gliding of the animal. 
As the animal grows it must of necessity move forwards 
within the rigid walls of the shell, since the increase in size 
takes place in every direction, quite as much in girth as in 
length. At the same time the soft visceral sac can accommo- 
date itself to a certain extent to straightened circumstances, 
sufficiently to avoid any sudden catastrophic movement, 
aud, meanwhile, gas is secreted by or through the thin septal 
area of the mantle, and when the limit of growth at any 
particular period is reached, a new system is laid down.” 
* Zool. Results, pt. vi. (1902), from which paper many of the following 
notes are taken, 
t Brooks, Proc. Boston Soc. Nat. Hist. xxiii, 1888, p. 380. 
Ann. G Mag. N. Hist. Ser. 9. Vol. ix, 35 
