318 Gasteropoda. -MOLLUSCA. Gasteropoda. 
to enable it to rise than the full unfolrling of its organs, 
horny cincture and muscles of adhesion from the rest 
and their protrusion from the shell. “ I incline rather 
of that cavity.” 
to the conclusion,” he says, “ that the sole functions of 
The shell of Nuutilm pompilius appears to have been 
the air chambers is that of the balloon ; and that the 
known to Aristotle, and is mentioned in sufficiently 
power which the animal enjoys of altering at will its 
precise terms by several of the ancient authors. It 
specific gravity must he analogous to that possessed by 
has long been used as an ornamental substance, and 
the fresh water testaceous Gasteropods, and that it 
the Dutch have been for a great length of time fond of 
depenas chiefly upon changes in the extent of the 
carving it with marine sketches — ships and dolphins, 
surface which the soft parts expose to the water. 
&c. In India, too, they ornament it in a similar manner. 
according as they may be expanded to the utmost. 
by carving and painting grotesque devices on its outer 
and spread abroad beyond the aperture of the shell, or 
surface, and converting them into elegant drinking cups. 
he contracted into a dense mass within its cavity. The 
Four species have been described, two of which. 
Nautilus may likewise possess the additional advantage 
however, have been considered by some naturalists as 
of producing a slight vacuum in the posterior parts of 
only the males of the other two. They are natives of 
the chamber of occupation, which is shut out by the 
the Indian Ocean, China Seas, and Persian Gulf. 
Class II.- GASTEROPODA (Univalves). 
The class of Gasteropods, the Univalves or Snails, are 
being inclosed in a cavity on the back, in some being 
what may be called the of the Mollusca. Of all 
lodged in a furrow between the mouth and the foot. 
the classes into which this sub-kingdom is divided, the 
and in others, being situated upon the back of the 
Gasteropods present in the most prominent and most 
animal, and floating freely in the water. In general 
marked degree the leading and essential features of 
the respiratory organs are atrophied on the left side. 
molluscous organization. The land snail and the 
though in some, as the Chitons, &c., they are repeated 
periwinkle will give a very good general idea of the 
on both sides. Usually, also, the Gasteropods are 
animals of this class. The animals have all a distinct 
unsymmetrical in conformation; the body instead of 
head furnished with eyes and tentacles, and when full 
being developed lengthwise, in a straight line, taking 
grown have a more or less distinct foot, placed under 
somewhat of a spiral direction, so that the mouth and 
the body, by which they are enabled to creep on the 
the excretory opening are brought in a slight degree 
ground or on the surface of the water. This foot in 
near to each other, instead of being placed at the tw'O 
some is a broad fleshy disc (see Murex), in others it is 
extremities of the body. 
narrow and formed like a furrow (see Strombus), or it is 
Their food is either vegetable or animal ; and the 
thin and presents the appearance of a slim plate placed 
mouth is provided with a denticulated tongue. “ This 
in a vertical position. This organ is for the most part 
tongue is muscular, and armed with recurved spines 
of a uniform structure throughout, its muscular fibres 
(or lingual teeth), arranged in a great variety of patterns. 
being interwoven with each other, and not collected 
which are eminentlj’’ characteristic of the genera. Their 
into distinct and separate hands. Some have it very 
teeth are amber-coloured, glossjq and translucent; and 
large, and it would appear that the larger and broader 
being silicious (they are insoluble in acid), they can be 
the foot, the slower is the motion of the animal, the 
used like a file, for the abrasion of very hard substances. 
narrower and more elongated it is, the quicker is its 
With them the limpet rasps the stony nullipore, the 
progress. They are, however, proverbiallj' slow of 
whilk bores holes in other shells, and the cuttle-fish 
pace, as every one must know who has watched the 
doubtless uses its tongue in the same manner as the 
march of the common snail ; and even it is one of the 
cat. The tongue, or lingual ribbon, usually forms a 
quickest walkers of the class. The heart is almost 
triple band, of wdiich the central part is called the 
alwaj's composed of a ventricle and an auricle, and its 
racMs, and the lateral parts pleurce ; the rachidian teeth 
position is near the back of the animal, but on the side 
sometimes form a single series, overlapping each other. 
opposite to that in which the reproductive organs are 
or there are lateral teeth on each side of a median 
situated. The arterial system is generally well developed. 
series. The teeth on the pleura: are termed undni; 
but the veins are imperfectly so ; the venous blood in 
they are extremely numerous in the plant- eating 
many of the species flowing through open spaces among 
Gasteropods. Sometimes the tongue forms a short 
the different organs. Some of the Gasteropods are 
semicircular ridge, contained between the jaw’s; at 
aquatic, while others are denizens of the dry land. The 
others it is extremely elongated, and when wdthdrawn ; 
organs of respiration, therefore, are constructed in such 
its folds extend backwards to the stomach. The 
a manner as to enable the animals either to breathe 
lingual ribbon of the limpet is longer than the whole 
atmospheric air or respire water. Those of the former 
animal ; the tongue of the whilk has one hundred rows 
group are provided with lungs, which have an external 
of teeth ; and the great slug has one hundred and sixty 
communication to admit air, opening under the outer 
rows, with one hundred and eighty teeth in each row. 
edge of the mouth. In the latter, respiration is carried 
The front of the tongue is frequently curved or bent 
on by means of branchiae or gills, the exact position of 
quite over ; it is the part of the instrument in use, and its 
which varies very much in different species ; in manv 
teeth are often broken or blunted. The posterior part 
