Blog 3: Starfish

Starfishes are found in the phylum enchinoderms. In this phylum we can also find feather stars, starfish, sea urchins, brittle stars, sea cucumbers, sand dollars and sea lilies. Starfish have radial symmetry, sometimes bilateral.They are sessile or slow-moving animals. The internal and external parts of the animal radiate from the center, which are often five spokes. A thin skin covers an endoskeleton of hard calcareous plates. Their body has more than two cell layers which has the tissues and the organs. Their body cavity is a true coelom. Most of them possess a through gut with an anus. Although we can see a starfishes' five arms we have no way of telling which one is the head. Their nervous system includes a circum-oesophageal ring. What is unique is their water vascular system. This is a network of hydraulic canals branching into extensions called the tube feet that function in locomotiom, feeding, and gas exchange. They have an open circulatory system but it is poorly defined. Sexual reproduction for these creatures usually involves separate male and female individuals that release their gametes into the seawater. This is known to be external fertilzation. The radial adults develop by metamorphosis from bilateral larvae. These creates do not have excretory organs, rather they rely on counter current exchange. 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.

Crinoidea:

Ophiocistioidea:

Astroidea:

Echinoiudea:

Holothuoidea:

SOURCES: http://www.earthlife.net/inverts/echinodermata.html
Campbell textbook.