AP Bio: Starfish

An Echinoderm.

Starfish are in the phylum of Echinodermata.  They have 5-rayed symmetry, usually radial but sometimes bilateral.  Their bodies have two cell layers of tissues and organs and the body cavity has a true coelom.  They do not have gills and instead have a open circulatory system.  They also have a water vascular system, operated by tube feet or feeding tentacles.  They normally reproduce sexually and are gonochoristic.  Starfish do not have excretory organs, but most have an anus.  Their nervous system includes a circum-oesophageal ring.  They feed through a mouth near the center of their body and feed on particles, detritus, or other animals.


These pictures below are examples of the living classes of Crinoidea, Ophiocistioidea, Astroidea, Echinoiudea, and Holothuoidea.


Crinoidea

This class includes sea lilies and feather stars.

Ophiocistioidea

A brittle star, this class is now extinct.

Astroidea

A sand dollar; this class includes sea stars, starfish, and sand dollars.

Echinoiudea

Sea urchins; this class includes sea urchins, sea biscuits, and sand dollars.

Holothuoidea

This class includes sea cucumbers, such as this one.

Sources:
information and Echinoderm picture from: http://www.earthlife.net/inverts/echinodermata.html
Crinoidea picture from: http://www.ucmp.berkeley.edu/echinodermata/crinoidea.html
Ophoicistiodea picture from: http://www.okc.cc.ok.us/biologylabs/documents/Echinodermata/Class_Ophiuroidiea.htm
Astroidea picture from: http://jbournesblog.wordpress.com/2011/12/13/the-gift-of-the-sand-dollar/
Echinoiudea picture from: http://ocean.nationalgeographic.com/ocean/photos/sea-urchins/
Holothuoidea picture from: http://animals.nationalgeographic.com/animals/invertebrates/sea-cucumber/