- Name Meaning: Spiny skin
- English Common Name: Echinoderms
- Major distinguishing characteristics: Five-fold radial symmetry, mesodermal layer, calcified spines
- Approximate number of species described: about 7,000 living species and 13,000 extinct ones
As adults, echinoderms are radially symmetrical marine animals. They occur at most depths within the ocean, and none are terrestrial or occur in freshwater. Additionally, all are predators.
They can reproduce asexually or sexually. Most species studied can also regenerate even a majority of their body. Essentially, if you chop up a sea star, you get more sea stars. Larval echinoderms are bilaterally symmetrical.
The Phylum Echinodermata includes 2 extinct and 3 living subphyla. The living subphyla include a total of 5 classes.
- Subphylum Homalozoa (extinct)
- Subphylum Crinozoa
- Class Crinoidea
- Class Edrioasteroidea (extinct)
- Class ?Arkarua (extinct)
- Class Cystoidea (extinct)
- Class Rhombifera (extinct)
- Subphylum Asterozoa
- Class Ophiuroidea
- Class Asteroidea
- Subphylum Echinozoa
- Class Echinoidea
- Class Holothuroidea
- Class Ophiocistioidea (extinct)
- Class Helicoplacoidea (extinct)
- Subphylum Blastozoa (extinct)
Echinoderms are radially symmetrical, usually with five point symmetry. In many-armed species this is usually show in multiples of five. They have two sides - an oral (mouth) side, and an aboral (opposite the mouth) side. These replace the ventral and dorsal orientation of vertebrates.
They are covered calcareous plates which arise from the mesoderm. Theses plates are made of calcite crystals, and have a sponge-like interior structure.
Echinoderms have a water vascular system which move nutrients around the body, and powers the tube-feet, allowing the animal to move. The system is open to the outside through a special pore called a madreporite.
The Fossil Record
Arkarua,from Precambrian Australia, may be the oldest known echinoderm. Echinoderms are certainly established by the Lower Cambrian about 500 million years ago. Because of their hard calcareous plates, they are well represented in the fossil record.