EAST. ZOOL. GAL.] NATURAL HISTORY. 
97 
with cross rows of hooks ; they have the faculty of dis¬ 
tending the body with water. The shell of the very 
young animal, (which may be seen on the tip,) is smooth, 
polished, with three or four gradually enlarging whorls, 
like a Helioc lucida ; it suddenly enlarges into the form of 
the adult shell. 
Here must be placed for the present, on account of 
the similarity of the form and texture of the shell, the 
Paper Nautilus ( Argonauta , Case 21) and probably the 
fossil genus Belleroption. As yet only a peculiar kind of 
cuttle fish, with a web to the end of the longer arms, 
has been found in the Argonaut shells; but there are 
many reasons for believing that this is only a parasite, 
adapted by its form to live in such shells, as the web 
of the arms is used by the animal to embrace the shell 
and keep it in its right position on the body; for, unlike 
all other mollusca which form the shell they inhabit, the 
cuttle-fish is not attached to the shell by any muscle, nor 
has the animal any muscle like the bone-bearing cuttle¬ 
fish, formed for the purpose of attaching the body to its 
internal shell. Secondly, the animal when alive does 
not fit the shell, so that the shell cannot have been moulded 
upon its body, as in other mollusca. Thirdly, the skin of 
the animal is of the same texture and appearance as the 
other naked cuttle-fish, and the presence of sand between 
the shell and the body appears to cause no uneasiness to 
the animal, as it does in all other shell-bearing mollusca, 
where the animal immediately rids itself of the irritation so 
caused, by covering it over with a calcareous coat. Lastly, 
the animals found in these shells are always females, 
and the apex of the shell is filed with very small eggs, 
while from the large size of the young shell which is 
to be seen on the apex of the true Argonaut, we should 
expect the animal which formed that shell, to have a large 
The other animals of this order have their branchiae 
placed on the right side of the body, in the groove between 
the edge of the mantle and the foot. They comprehend 
the two following families. 
The family of Pleurobranchidce (Case 21) have none or a 
very thin membranaceous shell inclosed in the mantle. The 
edge of the mantle and the foot project; the gills, which 
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