mantle; shell; nuchal membrane. 747 



Upon taking a shell, of which the animal had died and fiillen out, from a basket 

 at Lifu, something rattled like a stone in the shell. It turned out to be a handsome 

 pearl of large size, but of doubtful value as a gem, since the shape is not quite regular, 

 flattened on one side, and the surface is not absolutely pure. It measures about 

 15 millimetres in major diameter, 11 mm. in height, weighs 3690 milligrammes and is, 

 I believe, the first nautiline pearl to be recorded. 



The shape of the shell, a circinate spiral, belongs to one of those fundamental 

 forms which recur both primarily and secondarily under various guises in both kingdoms 

 of the organised creation. This form is calculated, for example, to meet the require- 

 ments of economy of space combined with strength and symmetry of construction, and 

 has therefore a physiological meaning. It is certainly remarkable to find a mollusc 

 provided with an external shell as large as that of Nautilus, which is at the same time 

 an expert and rapid swimmer. This faculty of swimming as the principal, practically 

 the sole means of locomotion, depends of course more upon the buoyancy of the shell 

 than upon its shape, but the latter should not be disregarded in this connection. 



The buoyancy of the shell is due to the series of air-chambers which have 

 long excited the admiration of poets and philosophers. Successive chambers are added 

 by the secretion of fresh septa pari passu with the growth of the animal. If the shell, 

 with the live Nautilus in it, be perforated over the chambers under water, the air 

 bubbles gently out as the water enters. I regret that I omitted to collect any of this 

 air, but Vrolik (1843) ascertained that it contained more nitrogen than atmospheric air, 

 a fact which is not suqjrising if we consider that the animal could better afford to 

 part with nitrogen than with oxygen. Nautilus seems to have a peculiar faculty of 

 producing gas', and my knowledge of the pallial veins was chiefly due to their automatic 

 injection with gas after removal of the animal from the shell. 



The chambers are not individually air-tight since they are perforated by the 

 siphuncle, 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 annulns 

 (girdle of Owen). Any loss by diffusion might be made good by the siphuncle, but 

 apart from this I see no reason to imagine that the air which fills the chambers 

 undergoes any appreciable fluctuations of pressure. It is, I am convinced, an error 

 to suppose that variations of pressure of the air in the chambers enable Nautilus to 

 rise or sink as the case may be. The air simply renders the shell buoyant once 

 for all. 



The progressive growth of the animal is accompanied by the addition of new 

 chambers, which give increased buoyancy and so maintain the shell under the entire 

 control of the animal until a limit is reached when the last air-chamber has been 

 divided off, and then only does propagation commence. 



' Professor Lankester (Article " Mollusea," Encyc. Brit., Vol. xvi. 9th edit.) says: — "In connexion with the 

 secretion of gas by the animal, besides the parallel cases ranging from the Protozoon Arcella to the Physoclistic 

 Fishes, from the Hydroid Siphonophora to the insect-larva Corethra, we have the identical phenomenon observed 

 in the closely-allied Sejiia when recently hatched." 



w. VI. 98 



