echinid

Echinids are a group of marine invertebrates belonging to the class Echinoidea. This class is part of the phylum Echinodermata, which also includes starfish, sea cucumbers, and brittle stars. Echinids are commonly known as sea urchins and sand dollars. They are characterized by their radial symmetry, typically with five parts, and a hard, spiny skeleton called a test. This test is composed of fused calcareous plates.

Sea urchins are generally spherical or globular in shape and are covered in movable spines. These spines can vary in length, thickness, and sharpness depending on the species and can be used for locomotion, defense, and sometimes even for feeding. They live on the seafloor, often in rocky areas, and graze on algae. Sand dollars, on the other hand, are flattened and disk-shaped, with shorter, finer spines that are almost bristle-like. They typically burrow in sandy or muddy substrates.

Echinids are found in all the world's oceans, from shallow coastal waters to the deep sea. Their diet consists primarily of algae, but some species may also consume small invertebrates or detritus. Reproduction in echinids is typically sexual, with external fertilization occurring when males and females release sperm and eggs into the water. The resulting larvae are planktonic before settling to the seafloor and metamorphosing into juvenile echinids. Their ecological role often involves controlling algal growth on reefs and providing a food source for other marine animals.