Gasteropoda. MOLLUSCA. Gasteropoda. 31 9 
of the lingual ribbon usually has its. margins rolled 
together and united, forming a tube, which is presumed 
to open gradually. Tire new teeth are developed from 
behind forwards, and are brought successively into use, 
as in the sharks and rays among fishes.” — ( Woodwm-d.) 
— See figures of Buccinum and Littorina — figs, 52 and 
57. When first hatched, the youthful Gasteropods are 
always provided with a shell ; but in many it is con- 
cealed when they reach the adult state, by a fold of 
the mantle, and in a number of species it is thrown 
off altogether at an early stage of their existence and 
never reappears.* 
The species which respire atmospheric air, the 
pulmoniferous mollusca, undergo no change as they 
approach maturity. When born they resemble their 
parents in all things but size. But those which respire 
water, the branchiferous mollusca, undergo a consider- 
able metamorphosis. They commence life in a small 
nautiloid looking shell, large enough to contain their 
whole body, and having the aperture closed by an 
operculum. The head is provided with a pair of 
ciliated arms- or fins of considerable size, which enables 
them to swim freely in the water, instead of crawling 
at the bottom. At this early period of their existence, 
all these water-breathers resemble each other, so that 
“ there is scarcely any difference between the young of 
eoUs and aplysia, or huccwum and Vermetus." — (Af. 
Edwards.) Development, however, soon commences; 
the mantle begins to secrete calcareous matter, and this, 
mixed with the epithelium or layer of animal matter, 
is moulded upon the body of the animal into the form 
which each genus is destined to assume. In form and 
general appearance the shells of the Gasteropods vary 
very much. They are usually, however, nearly spiral, 
though sometimes tubular or conical, and except in 
the family of Chitons, are composed of only one valve. 
The terminology used by conchologists, will be best 
understood by a reference to Plate 1, where the terms 
used are explained by the figures. 
The class Gasteropoda is a very extensive one, and 
its division into orders and families has engaged the 
attention of many naturalists. A variety of methods 
have been proposed, but it would be uninteresting to 
the general reader to enter into details here. We shall 
simply state the arrangement of Dr. Gray as adopted 
in our great national natural history repository, the 
British Museum. In his classical “ Introduction to 
Conchology,” Dr. Johnston thus expresses himself with 
regard to this system, “ There can, I think, be only one 
opinion as to the great merits of this method. Its 
superiority as a whole to any previous one can scarcely 
be questioned ; and the new views taken as to the 
position of several of the families in their respective 
orders, as well as in regard of the genera which are 
made to enter into the composition of the families. 
Tills is the case with the order Nudibranchiata (Gymno- 
hrancldata ) ; the larvre being provided with a shell and fins on 
the head, both of which are lost soon after development begins 
to progress, the shell most probably disappearing at the time 
the animals acquire a locomotive foot. 
nothing less than the most extensive and critical know- 
ledge of the entire class could have suggested.” 
The class G-asteropoda, Belly-walkers, or Snails, 
is divided into two large groups or sub-elasses, dependent 
upon the structure of their respiratory organs. In the 
first sub-class, the Ctenibranchiata, equivalent to 
the Prosobraechiata of M. Edwards, these organs 
are in the form of branchice or gills ; and they are 
comb-like, being formed of one or two series of lamellae 
or plates under or on the inner surface of the mantle 
on the back of the neck (rarely round its edges). In 
the species composing this group, the gill cavity is 
open, and both adults and larvte possess shells, the 
latter being provided, in addition to these, with deciduous 
ciliated fins on the head. They are either unisexual, 
or hermaphrodite and self-impregnating. In the second 
sub-class, the Hetekobranchiata, equivalent to the 
Opisthobranchiata of M. Edwards, the respiratory 
organs are variously formed; some having branchise 
or gills, which are exposed or only slightly covered by 
a fold of the mantle ; others having a simple lung, or 
a closed cavity lined with a network of respiratory 
vessels, like the lungs of beasts and birds. The species 
are all hermaphrodite, with reciprocal impregnation. 
The first sub-class, CTENOBRANCHiATA,is divided into 
two orders. Thefirst, Pectinibranchiata, is characterized 
by the comb-like branchiae or gills consisting of only 
one (rarely of two) longitudinal series of lamellae on 
the left side of the mouth, over the back of the neck. 
The gill cavity is open, the mantle edge being free from 
the back of the neck. The animals are unisexual, and 
the shells are spiral, andgenerally provided with a distinct 
operculum. The second order, Scutibrancliiata, have 
the gills consisting of two series of lamellae, forming 
one or two series over the back of the neck or under 
the edge of the mantle round the foot. The animals 
are hermaphrodite, self-impregnating, and the shells are 
conical, spiral, or symmetrical. 
The second large group or sub-class, the Hetero- 
BRANCHiATA, is divided into three orders. The first, 
Pleurobrancliiata, are characterized by the branchiae 
or gills forming a tuft on the side of the back, under a 
fold of the mantle. The animals are hermaphrodite, 
with reciprocal impregnation, and the shells are spiral. 
The adult and young are both shell bearing, and the 
latter are provided with deciduous cephalic fins. The 
animal and foot are elongate, formed for walking. The 
second order, Gymnobrancliiata (or Nudibranchiata), 
have the gills exposed or contractile into cavities on 
the surface of the mantle. The animals are heima- 
phrodite, with reciprocal impregnation, and the adults 
are destitute of shells, the larv^ only possessing that 
appendage. The third order, PneumobrancMata (or 
Pulmonata), differs from all the others in the animals 
respiring free air in a closed chamber lined with pul- 
monic vessels. The animals are hermaphrodite, with 
reciprocal impregnation, and both adults and larvae are 
shell bearing, the latter being formed like the parent 
and having no cephalic fins. 
