138 FIRST LESSONS IN ZOOLOGY 



removed (fij^f. 99). Around the mouth, which is at the cen- 

 ter of the under side, are five strong" teeth. Like the star- 

 fishes, the young sea-urchins are free swimming creatures 

 of very different appearance from tlic adults. Their food 

 consists of small marine animals and of bits of organic 

 matter which they collect from the sand and debris of the 

 ocean floor. Man)- of the sea-urchins are gregarious, 

 living togther in great numbers. Some have the habit 

 of boring into the rocks of the shore between tide-lines. 

 I have seen thousands of small, beautifully colored purple 

 sea-urchins lying each in a spherical pit or hole in hard 

 conglomerate rock on the California coast. How they 



Fig. 99. — " Test " of sea-tircliin, S/i-c'ii^y/ch\ii/iv/iis fnniciscmuis, with 

 spines removed. (From spet;imeii.) 



are enabled to bore these holes is not )-et known. There 

 is great variet}' in size and color among these animals. 

 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 the 

 sea bottom some sea-urchins, and when placed on the 

 ship's deck " 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 ver)- flat bod\- with 



