My Place!: Starfish

Starfish are often a pest of commercial clam and oyster beds, a single Starfish my eat over a dozen oysters or young clams every day. The body wall of echinoderms consists of three layers. The outer layer is only a single layer of cells which covers the entire animal including its various spines. The third layer is also a single layer of cells the main difference being that these cells are ciliated. This layer encloses the the animal's coelom separating the animals guts from its skin. The middle layer is much thicker and is called the dermis. It is composed of connective tissue and contains the exoskeleton. The exoskeleton supports the spines, warts and tubercles that are often found on the echinoderm surface.
Apart from its skin/spine, an echinoderm also has contact with the external world through its water vascular system and the tube feet that are a part of this system. The water vascular system of the echinoderms is unique in the living world and easily distinguishes them from all other phyla. The water vascular system starts with an opening to the external environment called a madreporite. From this a short straight canal called the stone canal leads to the ring canal. The ring canal is a ring as might be expected and it has five longitudinal canals branching off from it into each of the arms.
Echinoderms are either filter feeders, substrate eaters or carnivores. The gut is U-shaped in the Crinoidea with the mouth and anus being on the same surface. Echinoderms have a sub-epidermal nerve net running all over their body. As well as this they have a circum-oral nerve ring with 5 radial nerve cords extending from it into the arms. Many Echinoderms use their tube feet as organs for gaseous exchange, but others such as the Ophiuroidea and the Holothuroidea have additional special sites or organs of respiration.

Crinoidea