318 THE OCEAN WORLD. 
part enclosed in a shell, whence they are known as Testaceous 
Molluscs. They are called dzvalves, because their shell consists of 
two halves, or valves united by a hinge. They are sheltered in this 
double shell as a book is in its cover. 
Although they have no head, they can feed themselves, and they 
reproduce their kind. They have friendships and enmities, perhaps 
even passions—probably these are not yery lively, for most of them 
scarcely ever change their place, or even make the least movement ; 
many of them remain fixed to the rock on which they were hatched, 
and tumultuous sensations are not quite compatible with immobility. 
The bivalves* are found in every sea. The shells of the bivalve 
are ovoid, globulous, trigonal, heart-shaped, elongated like a pea-pod,: 
or flat like the leaves of a tree. In some one valve is flat, the other 
round and swelling in the centre. The shell is thus an outer 
envelope, consisting of two pieces, more or less corresponding to 
each other in size and shape (of which the oyster is an example), 
formed of carbonate of lime deposited in membranous cells in its 
outer layers, the inner layers being composed of thin coatings of 
lime deposited in the outer surface of the mantle. The valves are 
united to the animal by the insertion into them of certain muscles, 
and the mantle-lobes stretch over to the edges of the valves. The 
ligament which unites the two valves consists of a dense elastic 
substance, somewhat resembling india-rubber ; the hinge is formed 
of teeth in one valve and cavities in the other into which these 
teeth fit. The ligament acts in opposition to certain contractile 
muscles within, which draw the valves together, and is placed either 
within or without the hinge, or partly both. On separating the 
valves, the two folds of the mantle present themselves as thin 
delicate lamellz or leaves furnished at the margin with sensitive 
tentacles and other organs of sense, and with glands sometimes 
highly coloured. The use of these organs is thus described by 
Mr. Rymer Jones :— 
“When the animal is engaged in increasing the dimensions of its 
abode, the margin of the mantle is protruded and firmly adherent 
all round to the circumference of the valve with which it corresponds. 
Thus circumstanced, it secretes calcareous matter and deposits it 
upon the extreme edge of the shell, when the secretion hardens and 
becomes converted into a layer of solid testaceous substance. At 
intervals this process is repeated, and every newly-formed layer 
* The term bivalve, as constituting a class, must be taken in a limited sense, 
for several genera, as Pholas for example, have also accessory valves. 
