56 



The L'w'mg Things of the Earth unit i 



Fig. 79 The horny, rough tipper surface of a 

 C07HV1071 starfish. What kind of synnnetry has 



it? (AMERICAN MUSEUM OF NATURAL HISTORY) 



help in locomotion. Some of them are 

 brightly colored and are very beautiful 

 in structure. 



If you live near the sea, you are surely 

 acquainted with starfish, sea urchins, and 

 perhaps sand dollars. In tropical waters 

 the beautiful sea lilies, which you might 

 well mistake for plants, grow attached 

 to the sea bottom. All of these are spiny- 

 skinned invertebrates. 



The starfish. The starfish lives in salt 

 water near the shore. It is not a true fish, 

 of course. It is a living flexible star with 

 five arms and a spiny covering colored 

 brown or red or purple. Hundreds of 

 tiny tube feet with suction cups at 

 their ends dot the lower surface of the 

 animal. By pulling in and pushing out 

 the many tube feet in succession the 

 starfish moves along slowh' and 

 smoothly. These tube feet help in 

 breathing, too, and in food getting. By 

 folding itself over an oyster and at- 



FiG. 80 The sea urchin has a beautifully marked 

 shell beneath these spines. (American museum 



OF NATURAL HISTORY) 



taching its tube feet it pulls the shell 

 open. Then it turns its stomach inside 

 out and digests the living oyster in its 

 shell. 



Starfish do much damage by feeding 

 on mollusks. Oystermen formerly tried 

 to destroy starfish by tearing them in 

 half and throwing the pieces back into 

 the sea. Unfortunately, this made the 

 situation worse, for new parts similar 

 to those lost will grow back, or regen- 

 erate, making two animals where there 

 had been but one before. 



Some starfish relatives. Similar to the 

 starfish group but sufficiently different 

 to be put in another class are the sea 

 urchins and sand dollars. They, too, 

 have their mouths on the lower side. 

 They take in sand, in which thev find 

 small animals and plants which arc their 

 food. The sand dollar has a circular 

 flattened shell somewhat thickened in 

 the middle. The sea urchin is so covered 

 with movable spines that it looks Hkc 

 a walking pincushion. Sea urchins are 

 eaten by some people; their large masses 

 of eggs are considered a great delicacy. 



