124 ELEMENTARY ZOOLOGY 



are enabled to bore these holes is not yet known. 

 There is great variety in size and color among the sea- 

 urchins. The colors are brown, olive, purple red, greenish 

 blue, etc. 



A few kinds of sea-urchins have a flexible shell or test. 

 The Challenger expedition dredged up from sea-bottom 

 some sea-urchins, and when placed on the ship's deck 

 <4 the test moved and shrank from touch when handled, 

 and felt like a starfish. ' ' The cake-urchins or sand- 

 dollars are sea-urchins having a very flat body with short 

 spines. They lie buried in the sand, and are often very 

 brightly colored. Their hollow bleached tests with the 

 spines all rubbed off are common on the sands of both 

 the Atlantic and Pacific coasts. 



Sea-cucumbers (Holothuroidea). The sea-cucumbers 

 (fig. 23) show at first glance little resemblance to the 

 other radiate animals. The body is an elongate, sub- 

 cylindrical sac, resembling a thick worm or sausage or 

 cucumber in shape. At one end it bears a group of 

 branched tentacles which are set in a ring around the 

 mouth-opening. The body-wall is muscular and leathery, 

 but contains many small separated calcareous spicules. 

 There are usually five longitudinal rows of tube-feet. In 

 some species, however, tube feet are wholly wanting; in 

 others they are scattered over the surface. 



Although there are known about five hundred species 

 of sea-cucumber's many of which live along the shores, 

 they are much less familiar to us than the starfishes and 

 sea-urchins. They usually rest buried in the sand by day, 

 feeding at night. Some of them attain a large size. A 

 great orange-red species of the genus Cuciimaria, which 

 is found in the Bay of Monterey, California, is three feet 

 long. 



The people of some nations use sea-cucumbers as food. 

 They are called " trepang " in the orient. The trade of 



