MOLLUSCA. 199 



such passing prey as suits their appetite. Others, 

 equally incapable of locomotion, but furnished with 

 arms of different construction, (Brachiopoda,) catch 

 their food by similar efforts. The Tunicata, en- 

 closed in coriaceous bags, are firmly rooted to the 

 rocks ; or, aggregated into singular compound 

 masses, float at the mercy of the waves. The Con- 

 chifera inhabit bivalve shells ; while the Gastero- 

 pod orders, likewise defended in most cases by a 

 shelly covering, creep upon a broad and fleshy ven- 

 tral disk, and thus endowed with a locomotive appa- 

 ratus, exhibit senses of proportionate perfection. 

 The Pteropoda swim in myriads through the sea, 

 supported on two fleshy fins ; while the Cephalo- 

 pod Mollusca, the most active and highly organized 

 of this large and important division of animated 

 nature, furnished with both eyes and ears, 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."* 



In most of the Mollusca, the skin is developed 

 into a sort of mantle, which assumes various forms ; 

 it is often narrowed into a simple disk, formed into 

 a pipe, hollowed into a sac, or divided into fins. 

 Often in the thickness of the mantle a plate is 

 formed of different degrees of hardness, which seems 

 to bear some analogy to the spine of vertebrate ani- 

 mals. Generally, however, it becomes so large, that 

 the animal is able to shelter itself beneath it ; it 

 is then termed a shell, the covering skin of which 

 * Jones's Anim. Kingd. p. 351. 



