4. The _Limnaeidae_ embrace the "Pond-snails," chief of whom is the
well-known, high-spired _Limnaea stagnalis_. Other shells of this
family associated with _Limnaea_ are, however, very different in shape;
for instance, _Physa_ has its whorls turning to the left instead of to
the right; _Ancylus_ (Fig. 24), or the freshwater limpet, is of course
limpet-like; while _Planorbis_, or the "Coil-shell," is wound like a
watch-spring.
5. The _Auriculidae_ includes both spiral shells, such as _Auricula_
and _Charychium_, and a limpet-like one _Siphonaria_.
At the head of group _b_ stands 1, _Cyclostomidae_. _Cyclostoma
elegans_ is a common shell on our chalk-downs, and well illustrates
its family, in which the mouth is nearly circular. Foreign examples of
this genus are much esteemed by collectors. The other two families
are, (2) _Helicinidae_ and (3) _Aciculidae_.
Order III.--OPISTHOBRANCHIATA. These animals carry their gills exposed
on the back and sides, towards the rear of the body. Only a few have
any shell. 1. The _Tornatellidae_, which have a stout little spiral
shell. 2. The _Bullidae_, in which the spire is concealed (Fig. 26). 3.
The _Aplysiadae_, where the shell is flat and oblong or triangular in
shape. The remaining families are slug-like and shell-less.
[Illustration: Fig. 14. _Bulla ampulla._]
Order IV.--NUCLEOBRANCHIATA. Derives its name from the fact that the
animals constituting it have their respiratory and digestive organs in
a sort of nucleus on the posterior part of the back, and covered by a
minute shell. As they are pelagic, the shells are not readily to be
obtained. They are divided into two families, _Firolidae_ and
_Atlantidae_.
CLASS III.--PTEROPODA. Like the last, these pretty little mollusca are
ocean-swimmers. The members of one division of them, to which the
_Cleodora_ belongs, is furnished with iridescent external shells.
[Illustration: Fig. 15. _Petunculus guerangeri._]
[Illustration: Fig. 16. _Venus plicata._]
CLASS IV.--The LAMELLIBRANCHIATA (Plate-gilled), or CONCHIFERA
(Shell-bearing), includes the mollusca commonly known as "bivalves,"
the animal being snugly hidden between two more or less closely
fitting shelly valves. The oysters, cockles, etc., are examples of
this class. The two valves are fastened together near their points, or
beaks (technically called _umbones_), by a tough elastic ligament,
sometimes supplemented by an internal cartilage. If this be severed
and the valves parted, it will be found that in most cases they are
further articulated by projecting ridges or points called the _teeth_,
which, when the valves are together, interlock and form a hinge; the
margin of the shell on which the teeth and ligament are situated is
termed the _hinge-line_. A bivalve is said to be _equivalve_ when the
two shells composing it are of the same size, _inequivalve_ when they
are not. If the umbones are in the middle, the shell is _equilateral_
(Fig. 15); but _inequilateral_ when they are nearer one side than the
other (Fig. 16). If the shell be an oyster or a scallop, you will find
on the inside a single circular scar-like mark near the centre; this
is the point to which the muscles that close the valves and hold them
so tightly together are attached. In the majority of bivalves,
however, there are two such muscular impressions, or scars, one on
either side of each valve of the shell. The former group on this
account are often called _Monomyaria_ (having one shell-muscle), and
the latter _Dimyaria_ (having two shell-muscles). In the last named
the two muscular impressions are united by a fine groove (or
_pallial-line_), which in some runs parallel to the margin of the
shell (Fig. 15), whilst in others it makes a bend in (_pallial-sinus_)
on one side of the valve towards the centre (Fig. 16). In Monomyaria
it will be found running parallel to the margin of the shell. It marks
the line of attachment of the mantle or shell-secreting organ of the
animal to the shell which grows by the addition of fresh matter along
its edges, so that the concentric curved markings so often seen on the
exterior correspond in their origin with the periodic mouths of the
Gasteropods. The bivalves are all aquatic, and many bury themselves in
the sand or mud by means of a fleshy, muscular foot. These are
furnished with two siphons, or fleshy tubes, sometimes united,
sometimes separate, through which they respire, drawing the water in
through one and expelling it by the other. Those kinds whose habit it
is to bury themselves below the surface of the mud or sand are
furnished with long retractile siphons, and to admit of their
withdrawal into the shell, the mantle is at this point attached
farther back, giving rise to the _pallial-sinus_ above described; this
sinus is deeper as the siphons are proportionately longer, and in
many cases, too, the valves do not meet at this point when the shell
is closed.
Attention to these particulars is necessary when arranging your
bivalves, as on them their classification depends, the class being
divided into--
_a._ ASIPHONIDA (Siphonless).
_b._ SIPHONIDA _Integro-pallialia_ (with Siphons).--Pallial-line entire.
_c._ SIPHONIDA _Sinu-pallialia_ (with Siphons).--Sinus in pallial-line.