Starfish Description

Starfish are classified under the phylum Echinodermata. They exhibit 5 rayed radial symmetry, or sometimes bilateral symmetry, characteristic that developed through evolution. The body wall consists of three germ cell layers. The outer layer, the epidermis,  is a single layer of cells that covers the entire animal including its various spines. The middle layer, the dermis, is much thicker. This layer is composed of connective tissue and contains the organism's exoskeleton. For Starfish, the exoskeleton is a set of free moving small plates called ossicles. The third layer is also a single layer of ciliated cells that encloses the animal's coelom, separating the animal's internal organs from the skin. This is related to the phylum's characteristic of having a body cavity of a true coelom. The third layer creates what is known as the "coelomic lining".
Starfish possess a through gut with an anus with a headless body that can vary in shape. They have a poorly defined open circulatory system but a fairly good nervous system with a circum-oesophageal ring.  Starfish feed on fine particles of the water, detritus, or even other animals. Their diet includes oysters and clams and proves to make Starfish a pest of commercial clam and oyster beds. Interestingly, species of the phylum Echinodermata possess no excretory organs. Species of Starfish can reproduce by means of sexual reproduction and gonochoristic reproduction.
There are five living classes:
Crinoidea: Includes Sea lilies, Feather stars, and comatulids.

Image of Crinoidea structure from http://tolweb.org/tree/ToLimages/crinoid.gif

Ophiocistioidea: A class of extinct echinoderms from the Palaeozoic. They had a body structure similar to that of a sea urchin.

Image of Ophiocistioidea fossil from http://www.geomuseum.uni-goettingen.de/people/mreich/pdf/images/neuweb_seegurken11.gif

Astroidea: Harmless algal grazers and detrivores. Includes Starfish.

Image of Astroidea Starfish from http://www.reefs.org/hhfaq/starfish/star.jpg/view

Echinoiudea: Includes Sea urchins, which are spiny, globular animals. Sand dollars are closely related.

Image of Sea urchin from http://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Riccio_Melone_a_Capo_Caccia_adventurediving.it.jpg/250px-Riccio_Melone_a_Capo_Caccia_adventurediving.it.jpg

Holothuoidea: Includes Sea cucumbers, which have leathery skin and an elongated body containing a single, branched gonad.

Image of Sea cucumber from http://en.wikipedia.org/wiki/File:Espardenya_(animal).jpg