78 HOMES UNDER THE SEA. 



existence, are able to move at will through the water. 

 The sponges, barnacles, and oysters are familiar examples 

 of this fact. 



We find the same phenomena in these creatures. The 

 perfect Starfish or Urchin can only crawl very slowly, but 

 when it was a Pluteus, it could swim with considerable 

 rapidity. The long arms of the Pluteus are covered 

 with cilia, which, by their regular and successive vibra- 

 tions, drive the animal through the water. 



As the little creature proceeds in development, the 

 centre increases in thickness and density, the arms, with 

 their cilia, gradually disappear, their place being taken 

 by spines and ambulacra, and so the swimming Pluteus 

 becomes a crawling Urchin. 



We now come to the last group of Echinoderms, shoAv- 

 ing how they are linked to the worms. 



On reference to the illustrations, Figs. G and 7, the 

 reader will see two remarkable objects, each having a 

 tuft of tentacles at one end. These are Synaptas, a 

 species of Holothure, and are chosen because in general 

 form they approach the worms. 



The Holothures are found in most of the warmer seas, 

 and many of them are highly valued as food, under the 

 name of Trepang, Sea Slug, or Sea Cucumber. 



They are prepared for sale by being boiled, pressed 

 and dried, and differ greatly in their value. To an 

 unskilled eye they are scarcely more inviting than so 

 many scraps of shoe leather, and all look very much 

 alike. Yet all are important articles of commerce, and 



