THE ECHINODERMS 113 



in most cases into free-swimming, ciliated larvae, very 

 different from the mature forms. There are different 

 types of larvae characteristic of the different groups of 

 echinoderms. In the Ophiurans and sea-urchins the com- 

 mon larval form is furnished with long, ciliated arms 

 and is known as a pluteus. After a short, free life the 

 larvae settle down and undergo a complicated metamor- 

 phosis in changing into the mature form. 



