Echinoderms (Echinodermata) are a group of marine invertebrates that includes star fish, sea lilies, feather stars, brittle stars, sea cucumbers and sea urchins. There are about 6000 species of echinoderms alive today. Most echinoderms are bottom-dwelling animals that exhibit a variety of feeding habits including filter feeding, scavenging and predation. Present-day echinoderms are free-moving, but they evolved from sessile ancestors.
Echinoderms have an endoskeleton composed of calcareous ossicles. In sea stars and brittle stars, the ossicles articulate to form flexible structures. In sea urchins and sand dollars, the ossicles are fused together to form a rigid skeletal structure known as a test.
Echinoderms exhibit a type of radial symmetry called pentamerous symmetry in which their body can be divided into five equal parts around a central axis. Echinoderms developed this symmetry after they had already diverged from their bilaterally symmetrical ancestors. For this reason, the radial symmetry in echinoderms does not imply they are closely related to other organisms with radial symmetry such as cnidarians.
Echinoderms are divided into the following basic groups:
- Sea stars and starfishes (Asteroidea) - There are about 1500 species of sea stars and starfishes alive today. Members of this group live on the seabed and are found in every ocean around the world, from the intertidal zone down to the abyssal zone.
- Feather stars and sea lilies (Crinoidea) - There area about 600 species of feather stars and sea lilies alive today. Members of this group live attached to the seafloor and have a mouth at the top of their body that is encircled by feeding apendages.
- Sand dollars and sea urchins (Echinoidea) - There are about 950 species of sand dollars and sea urchins alive today. Members of this group have a round, spiny shell-like structure know as a test.
- Sea cucumbers (Holothuroidea) - There are about 1300 species of sea cucumbers alive today. Members of this group have a long body that is covered with leathery skin.
- Basket stars, brittle stars and snake stars (Ophiuroidea) - There are about 2000 species of basket stars, brittlestars and snake stars alive today. Members of this group have five long, thin arms.