How are sea urchin and brittle star pluteus larvae alike?

The pluteus larvae of sea urchins (echinopluteus) and brittle stars (ophiopluteus) are remarkably similar, representing a complex and advanced larval type within the echinoderm phylum.1

These larvae share several key characteristics:

• Structure: They are often described as resembling an artist’s easel turned upside down.1
• Support: Their bodies feature fragile arms formed by lobes of ciliated bands, which are structurally supported by delicate rods made of calcite.1
• Arm Count: While the number can vary, both types typically possess four pairs of arms.1

Because of these shared larval traits, some scientists suggest that sea urchins and brittle stars may form a natural evolutionary group, a classification that is also supported by certain biochemical studies.2