294 ANIMALS WITH SPINY SKINS— THE ECHINODERMS 



tive season and the sperms find the eggs and fertilize them. A little bi- 

 laterally symmetrical larva soon develops. Bands of cilia around it 

 enable it to swim freely in the water, and this aids in distribution. The 

 adult starfish is so sluggish that it does not get very far during its life- 

 time and if all its offspring grew up in the same spot conditions would 

 very likely be quite crowded, but this free-swimming larva can travel to 

 new locations and thus avoid the congestion. 



Regeneration of the starfish is rather extensive. A single arm, as 

 long as it contains a portion of the central disc attached to it, can regen- 

 erate all the missing parts. Another interesting process called autotomy 

 is illustrated by the starfish. If a portion of an arm is crushed, the 

 entire arm may be automatically severed from the body near the central 

 disc and a new arm regenerated. 





Photo by Winchester 

 Fig. 20.4. The brittle star. 



Brittle Stars — Ophiuroidea 



This class includes forms that rather closely resemble the starfish, but 

 generally are called brittle stars because their arms come off so easily. 

 If you merely grasp one of the arms with your fingers it is very likely 

 that the animal will cut off the arm by autotomy and crawl away and re- 



