36 STAR-FISHES, SEA-URCHINS, ETC. 



Class 11. STAR-FISHES (Asteroidea). 



The star-fishes assume the most varied and curious 

 shapes. In Sand-stars (Fig. 32), the body is a mere flat- 

 tened disk, the arms branching out suddenly, often round 

 and snake-like, while the feet have no suckers, and appear 

 from the sides of the rays instead of the bottom. The 

 Ophiacantha spimdosa is not merely a star in shape, but is 

 highly luminous. Some, as the Ophiothda, divide them- 

 selves spontaneously, the body looking as if it had been 

 chopped in two ; the two halves become separate indi- 

 viduals, new arms growing from the severed parts. In 

 one of the brittle stars, known as the basket-fish, the arms 

 are divided into many branches of twos bifurcating, as it 

 is called and resemble, when coiled, a bail of snakes. 

 They live in the coral reefs of the South, and are often 

 found off the New England coast. 



Development. The young are produced from eggs, some, 

 as Ophiocoma vivipara, appearing at once in adult form, 



FlG. 33. Development of common star-fish. A, free-swimming form ; 

 A', later stage settling on the bottom ; B, same assuming star- 

 shape. 



while others (Fig. 33) are at first minute sacs swimming 

 by aid of cilia, undergoing ma*ny changes, finally in two 

 or three years assuming the adult shape. The common 

 star-fish (Fig. 34) preys upon the oyster. 



