280 THE INHABITANTS OF THE SEA. 
Its two sail-like arms expanding in the air, and the six others 
rowing in the water, 
the keel of its ele- 
gant shell is pic- 
tured as dividing 
the surface of the 
tranquil sea. But 
as soon as the wind 
rises, or the least 
danger appears, the 
cautious argonaut 
takes in his sails, 
draws back his oars, 
creeps into his shell, 
and sinks instantly into a securer depth. Unfortunately there is 
not a word of truth in this pleasing tale. Like the common 
octopus, the argonaut generally creeps about at the bottom of 
the sea, or when he swims, he places his sails close to his 
shell, stretches his oars right out before him, and shoots back- 
wards like most of his class by expelling the water from his 
respiratory tube. 
As he sits loosely in his shell, he was supposed by some 
naturalists to be a parasite enjoying the house of the unknown 
murdered owner; but this is perfectly erroneous, as the young 
in the egg already show the rudiments of the future shell, 
and the full-grown animal repairs by reproduction any injury 
that may have happened to it. 
Thetetrabranchiate cephalopods, or Nautili, are very differently 
constructed from their dibranchiate relations. Here, instead of 
mighty muscular arms, furnished with suckers or raptorial claws, 
we find a number of small, sheathed, and retractile tentacles (/), 
surrounding the mouth in successive series, and amounting to 
little short of a hundred. ‘The head is further provided with a 
large muscular disk (gy), which, besides acting as a defence to the 
opening of the shell, serves also in all probability as an organ 
for creeping along the ground, like the foot in the Gastero- 
pods. The mandibles are strengthened by a dense calcareous 
substance fit to break up the defensive armour of the crustacean 
or shell-fish on which the animal feeds. There is no ink-bag, 
no organ of hearing, and the eyes (1) are pedunculated, and of a 
