STRUCTURE OP THE MOLLUSCA. 79 



In another order, which includes the snail and the greater 

 part of cockles, the foot, or instrument of motion, is placed 

 under the belly of the animal, and consists of a fleshy plate 

 or disk, protected underneath by a layer of a horny or calca- 

 reous substance, which, when the animal retreats into its 

 shell, serves to close up its opening. Their mantle is fixed 

 upon the back, and covers more or less of the body, the head 

 also being partly enveloped by it. The mouth has generally 

 a few tentacula or feelers beneath it, but they are sometimes 

 wanting. The eyes are very small, sometimes fixed to the head, 

 and sometimes situated upon the end of the tentacula ; but 

 they are also sometimes wanting. These animals are almost 

 always furnished with shells, which serve them as a residence. 



The Mollusca of another order, including the oyster, the 

 clam, the quahog, the muscle, and, in short, all the bivalve 

 shell-fish, have no apparent head, but only a mouth surrounded 

 by four tentacula, and situated beneath the folds of their 

 mantle. The mantle is generally composed of two folds, 

 which inclose the body between them, as a book is contained 

 within its covers. Sometimes the edges of the two folds are 

 united together, and form a complete sack. In the clam, 

 this sack terminates in a long, double, fleshy tube, which is 

 usually called the head of the animal, but in fact serves a 

 totally different purpose; one of the tubes being for the 

 entrance of the water which supplies the gills in respiration, 

 and the other serving as the termination of the intestinal 

 canal, and the mouth of the animal being situated at that 

 part of the body which corresponds to the other extremity of 

 the shell. 



The Giant Clam is the largest of the Mollusca, with a tes- 

 taceous covering. Its shell is more than three feet long, and ' 

 its body forms a meal for a great number of persons. It is 

 found in the Indian seas, and in different parts of the Pacific 

 ocean. 



Many of the animals of this kind are furnished with ah 

 organ denominated their foot, consisting of a fleshy mass 

 attached to their body, whose motions are produced like those 

 of the tongue of quadrupeds. This foot often gives rise to a 

 number of filaments or threads, by which the animal is capa- 

 ble of attaching itself to rocks or other marine substances 



finally killed. Its body was as big as a hogshead ; its arms, called its beards, were 

 as big as a man could clasp, and thirty feet long; and its cups or suckers held foiu 

 callous each. It weighed 700 pounds. The Kraken has been supposed t v be at 

 animal of the eame kind. 



