THE OCEAN WORLD. 



part enclosed in a shell, whence they are known as Testaceous 

 Molluscs. They are called bivalves, 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 very 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 lamellae 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 al^o accessory valves. 



