318 THE OCEAN WOULD. 



The true molluscs are divisible into two great classes : tlie Ace- 

 phalous, or Headless, and those having a head of structure more or 

 less perfect, which are called Cephalous Molluscs. 



The Acephalous or Headless Molluscs are so called from the Greek 

 a, privative, and Ke^aX^, head. They have no head ; the body is 

 surrounded by the folds of the skin ; the shell consists of two valves. 

 Such is a summary description of all the Acephalous Molluscs. They 

 are sometimes naked, and sometimes 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 carapace 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, even to 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 shell of the bivalve is 

 ovoid, globulous, trigonal, heart-shaped, elongated like a pea-pod, or 

 flat like the leaves of a tree, having an opening down the ventral side. 

 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 coat- 

 tings of lime deposited in the outer surface of the tissue, called the 

 mantle -leaves. The valves are united to the animal by the insertion 

 of certain muscles, and by the horny epidermis of the mantle, which 

 stretches over the edge of the valves. The hinge and ligament which 

 unite the two valves consist of a dense elastic substance, somewhat 

 resembling india-rubber ; the hinge is formed of teeth, and cavities into 

 which the teeth fit. The ligament acts in opposition to certain con- 

 tractile muscles within, which draw them together, and is placed either 

 within or without the hinge, or partly both. On separating the valves, 

 the two leaves of the mantle present themselves. These are thin 



* The term bivalve as constituting a class must be taken in a limited sense, for 

 several genera, as pholas for example, have six or more valves. . 



