ORDER TETRABRANCHIATA ; NAUTILUS. 353 



an inhabitant of the open sea, and possessing the power of sinking 

 at the slightest alarm. The general structure of the shell, which 

 may be taken as a type of the whole group of chambered shells, 

 will be evident from the accompanying figure ; which represents 

 it laid open. Externally it presents nothing remarkable, being 

 a flattened spiral ; but when its interior is examined, it is found 

 to be divided into chambers, by a large number of transverse par- 

 titions of shelly matter. The outer chamber is by far the largest, 

 and to this the body of the animal is restricted ; but it maintains 

 a connection with the rest by means of a membranous tube, called 

 the siphuncle (g. Fig. 548), which passes down through a per- 

 foration near the centre of each partition, and thus penetrates 

 even to the innermost and smallest chamber. Although the 

 history of the growth of the Nautilus cannot (for want of a 

 sufficient number of specimens) be positively stated, there is 

 every reason to believe that, at the usual period for the enlarge- 

 ment of the shell, the animal adds to the edges of the outer 

 chambers, in such a manner as at the same time to prolong and 

 widen it ; and that it then throws a new partition across its 

 lower or inner part, so as to form an additional chamber. Hence 

 the number of chambers would vary in different specimens, 

 according to their respective ages, and the consequent number of 

 additions they have made to their shells ; and this is found to be 

 the fact. 



895. The general structure of the animal is intermediate 

 between that of the Dibranchiate Cephalopods, and that of the 

 Gasteropods. The arms are very numerous, amounting to nearly a 

 hundred ; they are unprovided with suckers, and they are short 

 and slender, closely resembling the tentacula of many Gastero- 

 pods. The head supports a large fleshy disc ; upon which it is 

 believed that the animal can crawl over the bottom or shores of 

 the ocean, as a Snail upon its foot. The power which the 

 Nautilus possesses, of rising and sinking in water at will, has 

 been attributed to the chambered structure of its shell ; and to a 

 power which it has been supposed to possess, of diminishing its 

 bulk, by forcing water from the sac which surrounds the heart 

 into the siphuncle, or allowing it to be expelled from that tube 



