echinids

Echinids are a taxonomic class of marine invertebrates belonging to the phylum Echinodermata. This group, also commonly known as sea urchins, is characterized by a globular or flattened test, which is a rigid, spiny shell. The test is composed of numerous calcareous plates fused together. Echinids possess tube feet, which are small, flexible, and extensible appendages that they use for locomotion, feeding, and respiration. These tube feet are typically arranged in five double rows along the underside of the animal.

The spines of echinids are a prominent feature and vary greatly in size, shape, and color depending on the species. They serve a variety of functions, including defense against predators, aid in movement, and can even be used for burrowing. The mouth of an echinid is located on its underside, and it contains a complex jaw apparatus known as Aristotle's lantern, used for scraping algae and other food sources.

Echinids are found in all of the world's oceans, inhabiting a wide range of habitats from shallow intertidal zones to deep-sea trenches. They are primarily herbivores, feeding on algae and seagrasses, but some species are omnivores or detritivores. Reproduction in echinids is typically sexual, with external fertilization being common. Larval stages are planktonic before settling and metamorphosing into juvenile sea urchins.