NATURAL HISTORY. 
65 
GALLERY.] 
allied to these families. The viscera, as in the slugs, are chiefly con¬ 
tained in the elongated, transparent, gelatinous body, which usually ends 
in a tapering compressed tail; the heart and gills are protruded, forming 
a small dorsal mass, (which has been called the nucleus ,) and which in 
Pterotrachea 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 distending the body with water. The shell of the very 
young Carinaria (which may be seen on the tip) is smooth, polished, 
with three or four gradually enlarging whorls, like a Helix 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 Bellerophon. 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 w 7 hen 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 the sand, &c., wfith 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, wdiile from the large 
size of the young shell which is to be seen on the apex of the true 
Argonaut, w T e should expect the animal which formed that shell, to have 
a large egg. 
The other animals of this order have their gills 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 Pleurobranchdle ( 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 consist of a series of pyramids 
formed of triangular plates, are placed between them; the head is pro¬ 
duced, and furnished with two or four tentacles. 
The family of the Umbrellim: ( Case 21) have a suborbicular, flat, 
hard, external shell. The genus Umbrella was formerly supposed to 
have its shell placed on its foot, and was called Gastroplax, but this has 
been proved to be an error. Tylodina differs in the head being pro¬ 
duced and bifid; while in Umbrella it is sunk into a deep cavity in the 
front of the foot. 
The order Gymnobranchiata ( Case 22) have naked gills of various 
forms placed on different parts of the back, or a series of plates placed 
