Starfish

Starfish Temporal range: Ordovician–Recent PreꞒ Ꞓ O S D C P T J K Pg N
Fromia monilis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Echinodermata
Superclass:	Asterozoa
Class:	Asteroidea De Blainville, 1830
Child taxa and orders
Infraclass Concentricycloidea Peripodida Superorder Forcipulatacea Brisingida Forcipulatida Superorder Spinulosacea Spinulosida Superorder Valvatacea Notomyotida Paxillosida Valvatida Velatida[1] † Calliasterellidae † Trichasteropsida[2]

Starfish, or sea stars, are Echinoderms of the Class Asteroidea.^[3]p35

All live in the ocean, on the sea floor. Many starfish live in deep water, others in shallow water. Some live in the intertidal zone, between low and high tide.^[4] They have five or more arms and can be quite large. The Sunflower sea star (Pycnopodia helianthoides) is the largest: fully grown, its arm-span is about a metre. This is a bit larger than the famous Crown-of-thorns starfish, which lives on coral polyps.

Although starfish are invertebrates, they do have a kind of skeleton. The bodies of starfish are composed of calcium carbonate plates, known as 'ossicles'. These form the endoskeleton, which takes on a variety of forms such as spines and granules.

They have a primitive nervous system, but not a brain. They also do not have blood, instead they use sea water to pump things around their bodies.

There are over 1500 different species of sea star. Most sea stars are predators. They eat mussels, clams, and other bivalves. Occasionally, they catch small fish.^[5][3]

1. ↑ Sweet, Elizabeth (22 November 2005). "Fossil Groups: Modern forms: Asteroids: Extant Orders of the Asteroidea". University of Bristol. Archived from the original on 2007-07-14. Retrieved 2016-05-31.
2. ↑ Knott, Emily (7 October 2004). "Asteroidea. Sea stars and starfishes". Tree of Life web project. Archived from the original on 2020-04-25. Retrieved 2013-05-10.
3. ↑ ^{3.0 3.1} Cite error: The named reference Nichols was used but no text was provided for refs named (see the help page).
4. ↑ At low tide they would be in rock pools.
5. ↑ Sweet, Elizabeth (2005-11-22). "Asterozoa: Fossil groups: SciComms 05-06: Earth Sciences". Archived from the original on 2007-07-14. Retrieved 2008-05-07.