Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and 展开

Musculoskeletal
The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin dermis and 展开

Reproduction
Sea urchins are dioecious, having separate male and female sexes, although no distinguishing features … 展开

Fossil history
The earliest echinoid fossils date to the Middle Ordovician period (circa 465 Mya). There is a rich fossil … 展开

Urchins typically range in size from 3 to 10 cm (1 to 4 in), but the largest species can reach up to 36 cm (14 in). They have a rigid, usually spherical body bearing moveable spines, which give the class the name Echinoidea (from the Greek ἐχῖνος ekhinos 'spine'). The … 展开

Trophic level
Sea urchins feed mainly on algae, so they are primarily herbivores, but can feed on sea cucumbers and a wide range of invertebrates, such as 展开

Injuries
Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines. These are a … 展开