Definitions

1. noun

An order of irregular sea urchins, characterized by a round body and long, thin, flexible spines, which are often used for defense and feeding.

“The marine biologist studied the actinoidea species in their natural habitat.”

2. adjective

Related to or characterized by sea urchins of the order Actinoidea.

“The actinoidea features of the fossilized sea urchin were fascinating to the paleontologist.”