124 
NATURAL HISTORY. ^EAST. ZOOL. 
appear to be most allied to these families. The viscera, 
as -in the slugs, are chiefly contained in the elongated, 
transparent, gelatinous body, which usually ends in a taper¬ 
ing compressed tail; the heart and gills are protruded, 
forming a small dorsal mass, (which has been called the 
nucleus ,) and which, in Fterotrachea and Firola is naked, 
but in Carinaria is covered with a very thin, keeled, 
concentrically waved, obliquely conical, compressed shell, 
having a very large triangular mouth. These animals swim 
on the calm ocean with the back downwards; the mouth 
is large, provided with a cartilaginous tongue, and armed 
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 
count of the similarity of the form and texture of the 
shell, the Paper Nautilus ( Argonautu ) and probably 
the fossil genus Bellerophon . As yet only a peculiar 
kind of cuttle flsh, with a w T eb 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 filled with very small eggs, 
while from the large size of the young shell which is to be 
