MOLLUSCS I07 



It is common down to a depth of 300 fathoms and is 

 gregarious, moving about in areas where shrimps 

 abound. 



Normally it crawls by means of its tentacles, and these 

 vary in number according to sex and comprise a score 

 or more pairs, each tentacle being without suckers and 

 enclosed in a sheath. The animal is striped or mottled, 

 and this uniform, together with the zebra-markings of the 

 shell, make it very inconspicuous, particularly when seen 

 against a ripply-marked bed of sand. 



In an adult of the largest species the shell is divided 

 into some thirty odd chambers, filled largely with a 

 nitrogenous gas. Despite this buoyancy, the animal 

 lives chiefly on the sea-bed, and the mouth of the shell 

 is closed by two large, flattened, tentacled sheaths, which 

 unite to form a sort of lid or operculum. 



Apart from the Nautilus's very restricted use as food, 

 the shell is used in the manufacture of pearl buttons. 

 Very little is known as to the reproduction of the animal. 

 The eggs are relatively large, enclosed in elaborate capsules, 

 and are deposited in rows. 



The Dibranchia contains the giants and dwarfs of the 

 race, its representatives being found in every sea and at 

 all depths. It is divided into numerous families, genera 

 and species. 



The strange little animal called Spirula has, like the 

 Nautilus, a chambered shell, but this is almost wholly 

 covered by the mantle and is small in proportion to its 

 make-up. Practically nothing is known of the animal's 

 habits, specimens being exceedingly rare. Its shells, 



