SOME LOWER ANIMALS 255 



open. The animal, however, has strong muscles on the in- 

 side with which it can hold the shell tightly shut. 



The shell is increased by the addition of concentric layers, 

 which can be seen on the outside. Many mussels have a 

 beautiful, iridescent lining to the shell— the mother-of-pearl. 

 This is used in making buttons and ornaments. On the 

 Mississippi and its tributaries there flourishes a large in- 

 dustry of collecting clams for their mother-of-pearl. In 

 some clams and other mussels true pearls are formed as an 

 unnatural growth between the shell and the fleshy mantle 

 lining it. The nucleus of a pearl is generally some foreign 

 body, such as a grain of sand. Let the children read about 

 the Pearl Oyster of the Indian Ocean. Fine pearls are often 

 found in fresh-water mussels. 



In comparison with these animals the common Oyster 

 should be studied. Get unopened oysters and make a general 

 comparison of structure with that of the clam. 



Many creatures are called "worms" which are not strict- 

 ly so at all, but the Earth-worm is a true worm. A brief 

 study of the segmental body, the bristle feet, the life habits, 

 and the uses of this creature should be made. Worms, 

 unlike the worm-like caterpillars and grubs of insects, do not 

 change into some other form. For comparison a study may 

 be made of the Leech, or Bloodsucker, another worm that is 

 abundant in sluggish brooks, in ponds and lakes. 



The Starfish and Sea-urchins, found on the seacoasts are 

 types of animals built upon the radial plan. The mouth is 

 at the centre, on the under side, and the various other organs 

 of the body radiate from this centre. Starfish have a number 

 of arms, on the underside of which are little, sucker-like feet, 

 with which the animal moves. Sea-urchins have a similar 



