The echinoderms have radial symmetry, which means it has 5 or more arms. A starfish has two cell layers, making them diploblastic. Germ layers are the primary tissue layers in starfish. The coelem is the body cavity of a starfish that forms in the last stage of sea star development. Circulation occurs in the coelem, vascular system, and the hemal system. The hemal system is mostly organized to distribute nutrients in the digestive tract. Starfish can either undergo asexual or sexual reproduction. Fertilization takes place externally, and the fertilized embryos become part of zooplankton. If they reproduce asexually, they undergo fragmentation. Starfish feed on mussles. Their waste is excreted either through the anus(if present) or else through the mouth. Echinoderms do not have a real brain, instead they have a network of interlacing nerves, which is underneath the skin. The ring nerves and radial nerves of a starfish balance directional systems.
Crinoidea
Ophiocistioidea
Astroidea
Echinoiudea
Holothuoidea
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