268 A YEAR IN SCIENCE 



called "mother-of-pearl." Pearl buttons and knife 

 handles are made from the shells of our fresh water 

 clams. Sometimes an irritation is produced by the 

 presence of some foreign substance, as a small worm 

 which embeds itself in the mantle. As a result of this 

 irritation masses of pearl are formed, frequently in 

 concentric layers, around the irritating substances. 

 These are pearls so much prized as ornaments. Almost 

 all of the mollusks live in the water. They are widely 

 used as food as well as for the commercial purposes 

 already referred to. 



Worms. You are familiar with this group from 

 general observation. It is easy, however, to mistake 

 the larvae of some insects for members of this group. 

 The bodies of worms, such as the common earth worm, 

 are composed of a great many segments. These segments 

 are very similar in structure. Short hair-like feet, not 

 jointed, are present 011 many of them. 



Echinoderms. These ani- 

 mals are found only in salt 

 water. Here belong the 

 starfish and the sea urchin. 

 The echinoderms are all 

 made on a plan of five. This 

 is very evident in the star- 

 fish, which has the five arms 



A starfish. 



radiating from a central 



disc. These forms usually have the body covered with 

 a skeleton bearing spines. 



