MOLLUSCA. 241 



lurnisli materials valuable in the arts, and many of 

 them are eaten by mankind. 



Various are the forms, and widely different tlie 

 relative perfection of the Mollusea as regards their 

 endowments and capabilities ; some, as the Polyzoa, 

 fixed to the surface of foreign bodies, entirely de- 

 prived of organs connected with the higher senses, 

 and imable to change their position, are content to 

 protrude at intervals their ciliated arms, and thus 

 entrap such passing prey as suits their appetite. 

 Others equally incapable of locomotion, but furnished 

 with arms of different construction (Brachiopoda), 

 catch their food by an equally curious mechanism. 

 The Tunicata enclosed in leather-like bags, firmly 

 rooted to the rocks, or aggregated into singular com- 

 pound masses, adorn the beach with their kaleidoscope 

 patterns, or float through the ocean at the mercy of 

 the waves. The Conchifera inhabit bivalve shells; 

 while the Gasteropod orders, likewise defended in 

 most instances by a shelly covering, creep about by 

 means of a fleshy disk, and thus being endowed 

 with a locomotive apparatus, exhibit senses of pro- 

 portionate perfection. The Pteropoda swim in 

 myriads tlu'ough the sea, supported by two fleshy 

 fins ; while the Cephalopoda, the most highly organized 

 of this large division of animated nature, furnished 

 with both eyes and ea}:s, and armed with formidable 

 means of destroying prey, become tyrants of the 

 deep, and gradually conduct us to the most exalted 

 type of animal existence. These different sections^, 

 which, constitute so many distinct classes, into which 

 the Mollusea have been divided by zoologists, may be 

 arranged in accordance with the following tabular 

 view : — 



M 



