53 
Class V. Lamellibranchiata. 
Plate XXI (right side). 
The Bivalves are distinguished from the Snails 
by possessing a shell formed of two valves which are 
smooth and iridescent on the inner surface. The outside 
consists of a dark brown horny integument. Between 
the two longitudinal layers lies a strong calcareous 
layer, composed of cells of lime placed close to- 
gether like those of a honeycomb. When we opena 
living mollusc, we first find a fold of integument lining 
the inside of each valve of the shell. This is tho 
mantle, which is followed by two layers or plates 
on each side, the gills. Between these again is the 
body of the animal, round which is folded the strong 
muscle called the foot, which is protruded from be- 
tween the shells for purposes of locomotion. No 
head is visible; the opening of the mouth is invisible ; 
and as the animals feed only on microscopic orga- 
nisms, there is no need for either jaws or teeth. 
On each side of the mouth stands a pair of three- 
sided tentacles, which are set with microscopic cilia 
like the gills and the inner surface of the mantle, 
which give rise to currents which supply the animal 
with food. Among the other organs we may mention 
the liver, and intestine; and the nervous and vas- 
cular systems &c. In most species, the sexes are 
separate, and they lay eggs which pass through a 
portion of their development between the gill-laminz. 
The valves are closed by one or two strong muscles, 
which leave their impressions upon them; and are 
opened by the elastic ligament at the back of the 
shell, which also serves to bind the valves together. 
The valves are frequently connected by a system 
of teeth or notches fitting into corresponding depres- 
sions on the other side. As the ligament exists after 
the death of the animal, all dead shells gape. All 
the Bivalves are water-animals, and most of them 
are marine. A few swim, but most creep on the 
ground, or bury themselves partially in it. Many 
fix their shells on the rocks, and some attach it by 
a band of silk-like threads, which is called a byssus 
or beard. In many bivalves two projecting tubes 
called siphons rise from a peculiar elongation of the 
hinder portion of the mantle. The Bivalves are 
classified by the presence or absence of these, in 
conjunction with the number of clasping muscles, 
and their impressions on the inside of the valves. 
I. Bivalves without siphons. 
Fig. d. Ostrea edulis, the Oyster, is familiar 
to everyone. The convex shell is fixed to the rock, 
and the flat shell covers it like a lid. There is no 
foot, for the animal is incapable of free motion. In 
the middle of the shell is the imprint of a single 
large clasping muscle. The animals assemble in 
colonies called oyster-banks. The oyster was a 
favourite food in prehistoric times, and is so still, 
but at present it is rather a delicacy than a staple 
article of food for our nore numerous populations, 
although it has been artificially reared since Roman 
times. This is partly effected by artificially 
stocking bays with the young brood, and partly by 
removing oysters from the natural beds to so-called 
“oyster-parks’’, where they are fattened. The most 
esteemed oysters are the English ‘natives’, and those 
of Normandy, Brittany, and Holstein , and in America, 
the closely-allied Virginian Oyster. They are not 
found in the Baltic Sea. The annual consumption 



of oysters for human food must amount to many 
millions, and oysters are also preyed upon by many 
marine animals, such as star-fish, shells, sponges &c., 
which work great hover among the oyster-beds. 
Fig. b. The Scal llop (Pecten maxtimus) is another 
well- known bivalve of delicate flavour, and its shells 
were formerly placed by pilgrims in their hats. The 
animals, which haye only one clasping-muscle like 
the oysters, swim actively about by opening and 
closing their valves. 
Fig. c. Spondylus gaedaropus is a Mediter- 
ranean species which differs from the Scallop by the 
long spines placed on the longitudinal ridges, and 
in its habits, as the right hand shell is never firmly 
closed. Thanks to the spines, the beautiful shell is 
frequently covered with a thick mass of dirt and Alge. 
Fig. e. The Pearl Mussel (Meleagrina marga- 
vitifera) is another valuable shell. It is chiefly found 
in the Indian Ocean and in the Persian Gulf, and is 
sought after for the precious excretions of the mantle 
called pearls. In China they attempt to accelerate 
the production of pearls by artificial injuries inflicted 
on the mantle. The inner layer of the shell, called 
mother-of-pearl is also an article of trade. These 
animals have two clasping-muscles, but the first is 
so small that its impression in the shell is hardly 
visible, 
Plate XXII. fig. a. MWadleus vulgaris is another 
East Indian shell. It is remarkable for 
like form; and is laminated 
oyster and the pearl-mussel. 
Plate XXI. fig. f. Penna nobilis attaches its 
shell to a rock by a reddish-brown tuft of threads, 
or byssus. From this, the Italians manufacture 
gloves, purses &c. The shells are sharply trian- 
gular, and gape behind. There is the impression 
of a small muscle in front, and of a large one at 
the back. 
Fig. h. The Mussel (Mytilus edulis) is found 
attached to wood and stone-work as well as to rocks 
in enormous masses in all the seas of Northern 
Europe. It is not so much eaten now as formerly, 
because it is sometimes poisonous. In this species 
also the impression of the front clasp-muscle ts smal- 
ler than that of the hinder one. 
Fig. In the Noah’s Ark Shell (Arca Noe) 
the clasp-muscles and their impressions on the shell 
are of equal size. The thick shell exhibits very 
complicated toothing on the margins. 
The Fresh-water Mussels 
of the bivalves without siphons. They are found in 
rivers, brooks and ponds, and generally rest with 
the front buried in the mud, or creep slowly at the 
bottom of the water. The shells are oval, and the 
valves are of similar size and shape, and are covered 
externally with a strong brown epidermis, and are 
lined within with mother-of-pearl. The shape of the 
shells varies extremely according to locality, and 
the two genera are distinguished by the structure 
of the hinder end of the shell. 
its hammer- 
like the shells of the 
oO 
g. 
form the last family 
Fig. i. Anodonta anatina has a thin brittle 
shell, and the edge is straight, and not toothed. 
Fig. k. Unio pictorum has a coarser shell, and 
ridge-like teeth on the edges. One of the commonest 
allied species is Unzo margaritifera, which grows to 
a large size, and sometimes contains pearls. 
