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Characteristics of Echinoderms: Spiny-Skinned Animal
Echinoderm means “spiny-skinned animals” that constitutes a phylum with a common name for about 6000 living species of marine animals that make up Phylum Echinodermata such as sea urchins, sand dollars, starfish, brittle star and sea cucumber. They have five-part superficial radial symmetry and equipped with odd tube feet. They move slowly with less complex behavioral pattern.
The name of this phylum is derived from the Greek word “Echinos” which means hedgehog and “Derma” which means skin. You will wonder if what a hedgehog is; a hedgehog is an animal with rows of spines.
The scientific classification of echinoderms is divided into two subphyla however 20 extinct classes are recorded in fossil record:
- Class Crinoidea (Pelmatozoa) - Sea lilies and feather stars with 650 species.
- Class Holothuroidae (Eleutherozoa) - Sea cucumbers with 900 species.
- Echinoidea – Known as sea urchins and sand dollars with 850 species.
- Asteroidea – Commonly known as starfish, with about 1500 species.
- Ophiuroidea – also called brittle stars with about 2000 species.
Echinoderms are abundant in ocean floor at all depths and often make up a large group of living materials such as:
- Grazers – Mostly sea urchins
- Feeders – Brittle stars
- Predators – Starfish
The starfish is a typical echinoderm with a mouth surrounded by five arms with fleshy tube feet that helps the animal clings and crawl. These tube feet are supported by a complex internal hydraulic system that is pumped up by sea water.
Brittle stars swim and crawl by moving its arms while sea urchins don’t have arms, but have rows of tube feet that is called spines which is extended along the body surface. Brittle star spines are not vicious, however; it is not good to hold it for it may cut your hands. It escapes from danger as adaptation and when one of its arms got cut it grows again. Starfishes with slender arms also have the power of autotomy and regeneration.
- Autotomy - The self-cutting of the body part
- Regeneration – is the growing of a part
The sea urchin skeleton is made up of calcium carbonate that forms a large portion of the sea urchins body. Since the body is made up of outer skeleton it is called exoskeleton. It has also a well-developed gut with simple circulatory and nervous system.
Sea urchins are the spiniest of all echinoderms. In Philippines, there are several types of sea urchins:
- Black Sea-Urchin – with short spines and its egg is often eaten as delicacy
- Purplish Sea-Urchin – with relatively small body and very long spines and is very dangerous because it spines contains poison.
- Club Urchin – has stout spines which some people make into door-chimes because it creates pleasant sounds.
Sea cucumbers are echinoderm which is not spiny-skinned. A sea cucumber has an ovoid body that is same to the shape of a real cucumber, when the tentacles around the mouth are withdrawn. It has a habit of shooting out milky and sticky fluid when it is threatened. And if it hits you, you would have a hard time removing it from your clothes. The sea-cucumber uses it both for attacking and defending itself from its attacker.
Another distinctive characteristic besides the spiny-skeleton of echinoderms is the presence of multiple tube-feet at the ventral side of its body. Their main purpose is for movement, the two rows of tube feet at the ventral side of the arm of a starfish.
References ; Wikipedia, the free encyclopedia , Science and Technology by Lilia M. Rabago Ph. D , Crescensia C. Joaquin Ph.D, Catherine B. Lagunzad , PH. D, Encarta