ORDER TETRABRANCHIATA ; NAUTILUS. 353 



as a type of the whole group of chambered shells, will be evident 

 from the accompanying figure ; which represents it laid open. 



g 



FIG. 648. PEARLY NAUTILUS, with the shell laid open : t, tentacula ; e, funnel ; p, hood ; m, 

 portion of mantle ; o, eye ; g, siphon. 



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 partitions 

 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. 648), 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 

 chamber, 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 



