Introduction to Class Echinoidea

Purple Sea Urchin, Sphaerechinus granulari
Borut Furlan/WaterFrame/Getty Images

The Class Echinoidea contains some familiar marine creatures - sea urchins and sand dollars, along with heart urchins. These animals are echinoderms, so they are related to sea stars (starfish) and sea cucumbers.

Echinoids are supported by a rigid skeleton called a "test," which is made up of interlocking plates of a calcium carbonate material called stereom. Echinoids have a mouth (usually located on the animal's "bottom") and an anus (usually located on what could be termed the top of the organism). They also may have spines and water-filled tube feet for locomotion.

Echinoids may be round, like a sea urchin, oval- or heart-shaped, like a heart urchin or flattened, like a sand dollar. Although sand dollars are often thought of as white, when they are alive they are covered in spines that may be purple, brown or tan in color.

Echinoid Classification

Echinoid Feeding

Sea urchins and sand dollars may feed on algae, plankton and other small organisms.

Echinoid Habitat and Distribution

Sea urchins and sand dollars are found all over the world, from tide pools and sandy bottoms to the deep sea. Click here for some photos of deep sea urchins.

Echinoid Reproduction

In most echinoids, there are separate sexes and individual animals release eggs and sperm into the water column, where fertilization occurs. Tiny larvae form and live in the water column as plankton before eventually forming the test and settling to the bottom.

Echinoid Conservation and Human Uses

Sea urchin and sand dollar tests are popular with shell collectors. Some species of echinoids, such as sea urchins, are eaten in some areas. The eggs, or roe, are considered a delicacy.

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Kennedy, Jennifer. "Introduction to Class Echinoidea." ThoughtCo, Aug. 26, 2020, Kennedy, Jennifer. (2020, August 26). Introduction to Class Echinoidea. Retrieved from Kennedy, Jennifer. "Introduction to Class Echinoidea." ThoughtCo. (accessed June 10, 2023).