THE MOLLUSKS 



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and tear it in pieces by means of its horny, parrotlike beak. It is pro- 

 tected from its enemies and is enabled to catch its prey because of its 

 ability to change color quickly. In this way the animal simulates its 

 surroundings. The cuttlefish has an ink bag near the siphon which 

 contains the black sepia. A few drops of this ink squirted into the water 

 may effectually hide the animal from its enemy. 



To this group of animals belongs also the octopus, or devilfish, a ceph- 

 alopod known to have tentacles over thirty feet in length, the paper 

 nautilus and the pearly nautilus, the latter made famous by our poet 

 Holmes. 



/ Habitat of the Mollusks. — MoUusks are found in almost all parts of 

 the earth and sea. They are more abundant in temperate localities than 

 elsewhere, but are found in tropical and arctic countries. They are found 

 in all depths of water, but by far the greatest number of species live in 

 shallow water near the shore. The cephalopods live near the surface of 

 the ocean, where they prey upon small fish. The food supply evidently 

 determines to a large extent where the animal shall live. Some moUusks 

 are scavengers ; others feed on living plants. 



We have found in the forms of mollusks studied that almost all mol- 

 lusks Uve in the water. There is one great group which forms a general 

 exception to this, certain of the snails and slugs called pulmonates. But 

 even these animals are found in damp localities, and at the approach of 

 drought they become inactive, remaining within the shell. The Euro- 

 pean snail {Helix pomatia) imported to this country as a table delicacy 

 exists for months by plugging up the aperture to the shell with a mass of 

 slimy material which later hardens, thus protecting the soft body within. 

 ^ Economic Importance. — In general the mollusks are of much economic 

 importance. The bivalves especially form an important source of our food 

 supply. Many of the 

 mollusks also make up 

 an important part of the 

 food supply of bottom- 

 feeding fishes. On the 

 other hand, some mol- 

 lusks, as nalica, bore into 

 other mollusk shells and 

 eat the animal thus at- 

 tached. Some boring 

 mollusks, for example 

 the shipworm ( Teredo 

 navalis), do much dam- 

 age, especially to 

 wharves, as they make 

 their home in piles. Still others bore holes in soft rock and live there. 



The shells of mollusks are used to a large extent in manufactures and 



Piece of timber, showing holes bored by the shipworm. 



