Sea urchins or urchins are small, spiny

Spine (zoology)

A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

 phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 ) across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

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