30 CUTTLE-FISHES. NAUTILUS. 



appendages arranged like the calmaries; but their body, which 

 is oval and depressed, is furnished with fleshy, lateral fins, occu- 

 pying the whole length of the sack, and the back is sustained by 

 a large internal shell, the structure of which has been already 

 mentioned. The species most commonly found in the seas of 

 Europe Sepia officinalis attains a foot in length or more. 

 The cuttle-fish bone or shell, in powder, is used as a dentifrice, 

 and is employed in the arts for several purposes, as for polishing, 

 for forming moulds for silver castings, and as a pounce. 



24. The family of CEPHALOPODA TETRABRANCHIATA has for 

 its type the NAUTILUS, a very remarkable mollusk, the body of 

 which is enclosed in the last chamber (a) of a large shell, folded 



spirally, and divided by transverse 

 partitions (b) into several cavities, 

 (Jig. 15). Each one of these par- 

 titions is pierced by a hole, and the 

 canal thus formed, which is called 

 the syphon (s), extends to the pos- 

 terior extremity of the shell; it is 

 traversed by a contractile mem- 

 branous tube, posterior to the body 

 of the animal. This structure 

 b c seems designed to facilitate the as- 

 Fig. 15. SECTION OF A cent or descent of the animal in the 

 NAUTILUS. water, by increasing or diminishing 



the specific gravity of the shell ; for the syphon communicates 

 with an external reservoir, and can empty or distend itself with 

 the water found in it ; now, the chambers which it passes through 

 are filled with air, and when this tube becomes inflated, it must 

 compress this elastic fluid, and increase its density, which, at the 

 same time, augments the specific gravity of the whole shell, and 

 must give it a tendency to sink towards the bottom of the water 

 in which it floats. 



25. The conformation of the animal differs very much from 

 that of the poulpes, calmaries, and other dibranchial cephalopods. 

 The head of the nautilus is surrounded by a large fleshy disk, 

 which bears some analogy to the foot of the gasteropods, and 



Explanation of Fig. 15. Vertical section of the nautilus shell : a. the 

 last chamber, in which the animal is lodged ; 6. one of the partitions which 

 divides the chambers (c) from each other j s. the syphon. 



24. What are the peculiarities of the nautilus shell ? What seems to be 

 the object of this peculiar structure ? 



25. How is the animal of the nautilus characterized ? 



