HARDWICKE'S SCIENCE-GOSSIP. 



27 



•was geueraliy supposed that it was an exterior 

 shell very much like that of the Nautilus in its 

 function. Such, however, was found not to be the 

 case, for a living specimen was lately procured by 

 Mr. Percy Earl on the coast of New Zealand, which 

 proved that in this cuttle the shell is contained 

 within the mantle, and is in no part external. 

 There are three different species of Spirula, differing 

 from eacli other but slightly ; they are all divided 

 into separate chambers connected by a siphuncle. 



f^) 



S, 



' '''' Wh- c .f ^ ■ > ^ 



••'^'i/f'\J\. :lt'** 



Fig. 21. Ceratites ■iiodosiis, from the Muschcl-Kalk limestone, 

 showing; lobed chambers. 



We now come to the second order of cuttles, the 

 Tetrahranchiata, or "four-gilled." The animals of 

 this order are all protected by an external shell, 

 they progress in exactly the same manner as other 

 cuttles ; but their arms, which are very numerous, 

 are not furnished with suckers. Three families 

 only, the Nautiliche, Orthoceratidtc, and AmmonitidtP, 

 are contained within this order, many hundred dif- 



;^^- - ^? 



Fig. 22. Clymenia, from Devonian limestone, showing zig- 

 zagged chambers. 



ferent species of shells belonging to these three 

 families are known, but of these three only are 

 recent, all the rest being fossils. We are all well 

 acquainted with the shell of the Pearly Nautilus 

 {Nautilus pompilius), which will serve as a type for 

 its family. The shell of a Nautilus, when cut into 



>\ 



h 



two halves, appears to be divided into a number of 

 cells {septa), wliich are connected by a small tube, 

 the siphuncle. All the four-gilled cuttles have 

 shells similarly partitioned off, although in some the 

 shell is straight, in others only slightly spiral, and 

 others often coiled. The Nautilus is furnished with 

 a number of tentacles, which are of two kinds, those 

 about the mouth being of a different description to 

 those which serve as arms. It occupies the front 

 cavity of the shell, and can shut itself in by means 

 of two arms, to which is attached a leathery sort of 

 hood corresponding to the operculum of some uni- 

 valves. The other chambers which do not contain 

 the body of the animal, but are connected with the 

 heart by the siphuncle, which contains a mem- 

 branous tube exactly fitting all the cavities, are 

 used to float the animal. Although water could 

 not gain access to the cavities, because 

 the entire circumference of the mantle 

 in which the siphuncle originates is 

 firmly attached to the shell by a horny 

 girdle quite impenetrable to any fluid, 

 yet it is supposed that the chambers 

 can be filled with a liquid from the 

 pericardium, which compresses the air 

 already contained in them, and so the 

 centre of gravity is changed. By thus, 

 so to speak, shifting its balance, the 

 Nautilus rises to the surface or sinks 

 down to the depths at will. Owing 

 to the paucity of living specimens 

 examined by scientific men, but little 

 is positively known about the habits 

 of the Nautilus. Mr. G. Bennet, I 

 believe, was the first man of science 

 who had the good fortune to obtain a 

 living specimen. This gentleman was 

 in Mare Kini Bay, near Erremanga, 

 when a Nautilus was seen not very far 

 from the ship, floating on the surface 

 of the sea with the upper portion of 

 the shell raised above water, and kept 

 in a vertical position by means of the 

 included air, and, in the w"ords of the 

 sailors, looking very much like a dead tortoise- 

 shell cat in the water. On being captured 

 the upper portion of the shell got broken by the 

 boat-hook, as the animal was just sinking when 

 caught. The shell is so well known that a descrip- 

 tion of it would be superfluous ; but a few remarks 

 on its ingenious structure, formed so as to resist 

 the great pressure it would have to encounter when 

 at the bottom of the ocean, may be of some interest. 

 The shell is constructed in every way on the prin- 

 ciple of the arch, so as to offer the greatest resist- 

 ance to pressure, by making each part bear its 

 share of the weight. In some fossil species the 

 strength of the shell is greatly increased by its 

 being formed into ribs, thus fortifying it in a man- 



Fig. 23. 



Ancient 

 Bi^lemnite, 

 (restored). 



