Echinodermata

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Echinoderm, common name for about 6000 living species constituting a phylum of marine animals, such as starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers. They usually show a superficial five-part radial symmetry, and generally are equipped with peculiar tube feet. The phylum name is derived from the spiny skin.

An echinoderm such as the starfish typically has a mouth surrounded by five arms that bear minute, fleshy tube feet with which the animal clings and crawls. The tube feet are supported by a complex internal hydraulic system that is inflated with seawater. Some echinoderms, especially brittle stars, crawl or swim by moving their arms. Often, as in sea urchins, rows of tube feet extend along the body surface, and arms are absent. The spines are particularly well developed in sea urchins. The skeleton, made up of calcium carbonate, may form a large proportion of the body, or, as in some sea cucumbers, it may be greatly reduced. The fossil record shows that the five-rayed pattern is a late evolutionary acquisition, and deviations from it are common. The animals have a well-developed gut, but the nervous and circulatory systems are simple. Echinoderms are slow-moving and without complex behavior patterns.

Echinoderms are common on the ocean bottom at all depths; in the deep sea they often make up the bulk of living material. They may be grazers (most sea urchins), feeders on small particles (many brittle stars), or predators (most starfish). Starfish and a few others are pests, as when found in oyster beds. Sea cucumbers and sea urchins are eaten by humans.

Scientific classification: Echinoderms make up the phylum Echinodermata. The living echinoderms are commonly divided into two subphyla. The stalked or attached Pelmatozoa consist of the single class Crinoidea, or sea lilies and feather stars, with 650 species. The free-living Eleutherozoa consist of the classes Holothuroidae, or sea cucumbers, with 900 species; Echinoidea, or sea urchins and sand dollars, with 850 species; Asteroidea, or starfish, with about 1500 species; and Ophiuroidea, or brittle stars, with about 2000 species. Many authorities, however, unite Asteroidea and Ophiuroidea into a single class, Asterozoa (or Stelleroidea). About 20 extinct classes are found in the fossil record.

Types of Invertebrates

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Arthropoda -> Largest and most diverse invertebrate phylum characterized by animals with jointed limbs, a segmented body, and an exoskeleton made of chitin. Arthropods are abundant and successful in almost all habitats. The exoskeleton is divided into plates that enhance flexibility and movement. Periodic molting of the exoskeleton permits growth. Arthropods have a complex brain and nervous system. Many arthropods have a compound eye made up of numerous light-sensitive parts. Reproduction is sexual. – Examples: Ants, beetles, butterflies, lobsters, shrimp, crabs, scorpions, spiders, ticks

Ectoprocta or Bryozoa -> Small, mostly marine animals that resemble simplified horseshoe worms, with a lophophore surrounding the mouth. They live in colonies, attaching to the sea bottom or seaweed. Bryozoans are hermaphroditic and reproduce asexually (by budding) and sexually. – Examples: Moss animals

Entoprocta or Kamptozoa -> Small, mostly marine animals with a globular body mounted on a stalk. A lophophore surrounds both the mouth and anus. Entoprocts live in colonies. They reproduce both asexually and sexually. – Examples: Urnatella, Pedicellina

Brachiopoda -> Marine animals that resemble clams except that their shells form on the top and bottom of the animal, while clam shells form on the left and right of the animal. The shell attaches to the ocean bottom, rocks, or other objects by means of a cordlike stalk. Brachiopods reproduce sexually. – Examples: Lamp shells

Echinodermata -> Marine animals distinguished by their radial symmetry in which the body can be divided into five parts arranged around a central axis. They have internal skeletons composed of calcareous ossicles with projecting spines that give the body surface a bumpy appearance. They use body appendages called processes for feeding and locomotion. Reproduction is sexual and some have a larval form. – Examples: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers

Cycliophora -> Discovered in 1995, these parasites live in the mouthparts of certain lobsters. They have a characteristic saclike body with a bell-shaped mouth structure called a buccal funnel. Only one species has been identified, and it displays alternation of generations, with both asexual and sexual components of the life cycle. – Example: Single species identified as Symbion pandora

Loricifera -> Marine sediment dwellers with a protective structure called a lorica, consisting of plates that surround the animal's trunk. The mouth and head retract into the lorica for protection. Reproduction is sexual and little is known about the life cycle, although a larval form has been identified. – Examples: Nanaloricus mysticus

Onychophora -> Terrestrial, caterpillar-like animals found only in the tropics and southern hemisphere. The soft body is covered by a flexible cuticle that periodically molts. The head region contains a pair of antennae and clawlike mandibles. They have from 14 to 43 pairs of legs, depending upon the species and gender. Reproduction is sexual. – Examples: Velvet worms

Chaetognatha -> Torpedo-shaped marine animals with fins that enable them to swim with rapid, dartlike movements as well as gliding and floating motions. Movable hooks on their heads are used to capture prey. These animals are hermaphrodites and reproduction is sexual. – Examples: Arrow worms

Hemichordata -> Simple, wormlike marine animals with a primitive notochord and a system of gills. They use a distinctive proboscis to capture food and also to aid in locomotion. Reproduction is sexual and some have a larval form that resembles the larvae of echinoderms. – Examples: Acorn worms