THE MOLLUSCA. 115 



metry in the two halves of the body, and the absence of joints 

 or articulations, and lateral locomotive appendages. 



The majority of our Shells are divided into two classes 

 Bivalves and Univalves. The Bivalve is a shell in two parts, 

 a right and left valve, connected by a hinge. The Univalve 

 is a conical or spiral shell, often closed by an operculum, 

 which is a plate attached to the foot of the animal, corres- 

 ponding in shape to the mouth of the shell. " The Mollusca, 

 though nearly all sedentary in their habits, are in their 

 earlier stages swimming animals, being provided with cilia 

 which enable them to move freely about. Aided by these 

 and the ocean currents, they are dispersed, sometimes to 

 immense distances, until they meet with conditions suitable 

 to their growth. It is a remarkable fact that the Bivalves, at 

 this period of their lives, have eyes, to aid them in their 

 movements." Thus there is a natural means by which their 

 over accumulation in any particular part is prevented. After 

 a few days of this free and sportive life, they begin to settle 

 down to the conditions and localities each is destined to 

 occupy. The limpet attaches itself to the rock, between high 

 and low water mark ; the cockle, the mya, and the razor-fish 

 bury themselves in the sand and mud ; the Teredines attack 

 and burrow into the sides of ships or the hardest wood, and 

 by their silent and ceaseless operations undermine some of 

 the most important works of man ; the Pholas excavates itself 

 a home in the rocks and cliffs, by what means science has 

 failed to discover ; the mussel forms itself a byssus or cable, 

 by which it is attached to rocks and timber, and one species 

 spins itself a silken nest. Some tribes retain the power of 

 moving about ; the Pecten and the Pinna take flying leaps 



