Blog 3 Starfish

     The starfish is in the phylum Echinodermata, it has 5-rayed symmetry, mostly, radial, and sometimes bilateral. The first echinoderms did not have any radial symmetry showing hat this characteristic was acquired later in the group's evolution. a number of echinoderms are distinguished from other phyla because they have radial or occasionally bilateral symmetry. There are five planes of symmetry. The body of an echinoderm has three layers. The epidermis is the outer layer, and it is a single layer of cells which covers the entire animal. The middle layer is thick and is called the dermis. It is composed of connective tissue and the exoskeleton. The exoskeleton has three different forms; a set of closely joined plates with individual movement, a set of separately free moving small pieces  called ossicles, and a collection of widely separated microscopic ossicles lying in the dermis. The third layer is a single layer of cells that are ciliated. This layer encloses the animal's coelom separating the animals guts from its skin. The starfish has a poor defined open circulatory system. They reproduce normally sexual and gonochoristic. A starfish may eat over a dozen oysters or young clams a day. Echinoderms are filter feeders, substrate eaters, or carnivores. In the Crinoidea the gut in U-shaped and the mouth and anus on the same surface. In others it is straight- through gut with the mouth and anus on opposite sides of the body. The nerves that are running all over the body are in connection with those of the sub-epidermal nerve net. They also use tube feet as organs of respiration.

Crinoidea

 

 

 

 

Ophiocistioidea

Astroidea

Echinoiudea

Holothuoidea

Source: http://www.earthlife.net/inverts/echinodermata.html