of the Animal of the y^aufilouha and Aiiinionoidea. i>',)7 



It has been suggested that tlie formation of successive 

 8ei)ta in the shells ot" these animals is correlated with the 

 recurrence of reproductive periods; hut Dr. Wiley* states 

 definitely that this is not the case in Av/mZ/Ym*, since propaga- 

 tion, according to his observations, only Uikes place al'lcr the 

 last septum has been formed (p. 717_). 



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 f, but collectively they are rendered 

 an air-tight and v\ater-tight hydrostatic a[)paratus, owing 

 to the fact that tiie animal itself completely closes up liie 

 entrance to the chambers in virtue of its adherence to 

 the sliell by the muscles and annulus (girdle of Owen). 



The method of formation of the sejita, Dr. Willev writes 

 (p. 7-19), '•' 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 l.odily 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 

 suljstance of the hinder border, coincident with a foiniation 

 of fresh muscular substance in front. But this pretended 

 resorption of muscle-fibres could not be confined to the ends 

 of the muscles, where tliey abut upon the shell, but must 

 afi'ect the entire body of the muscles. There is no evidence 

 whatever that anything of the kind takes place, since the 

 muscles increase in size j)ari 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 clearlv 

 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 circunistancvs, 

 sufficiently to avoid any sudden catastrophic movement, 

 and, meanwhile, gas is secreted by or through the thin septal 

 area of the mantle, and when the limit of growth at anv 

 particular period is reached, a new system is laid down." 



* Zool. Kesults, pt. vi. (11)02), from which paper luauy of the following 

 notes are taken, 

 ■f Brooks, Proc. Boston Soc. Nat. Hist, xxiii, 1888, p. 380. 



Ann. c£- Mac/. X. Hist. Ser. 9. Vol. ix. 35 



