202 



METAZOAN PHYLA 



of rapid locomotion, but serpent stars are even more active, the rays 

 writhing about hke the tails of so many snakes when the animal is strongly 



Fig. 108. 



-A brittle star, Ophioderma sp. The oral surface. 



XV2. 



From a preserved specimen. 



stimulated. The basket stars are characterized by complexly branched 

 rays ending in tendril-like tips. They are found mostly in water of 

 considerable depth, clinging to masses of seaweed. Owing to the slender- 

 ness of the rays of ophiuroids, they are more likely to be broken than 

 are those of ordinary starfishes. Autotomy is also more frequent, while 

 regeneration is relatively rapid and complete. 





Fig. 109. — A sea urchin, Strongylocentrotus drohachicnsis (Miiller). From a pre- 

 served specimen. The spines have been stripped from the right half; they and the tube 

 feet show on the left. X %. 



236. Echinoidea. — Echinoidea, or sea urchins (Fig. 109), are ani- 

 mals which have lost their rays and possess a skeleton made up of rows 



