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Modern forms


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Asteroids

Modern asteroids are active and vicious predators existing on a diet of shellfish. Their mode of attack is an unusual one. With their sucker covered tube feet they can pry open bivalve shells just enough to evert their stomachs through their mouths. They force their stomach into the mantle cavity of the animal and digest the soft parts.

Extant Orders of the Asteroidea

Brisingida

Forcipulatida

Notomyotida

Paxillosida

Spinulosida

Valvatida

Velatida

Brisingida

Deep-sea dwelling asteroids. They usually have many, between 6 and 16, long, attenuated arms which they use for suspension feeding. The Brisingida contains about 100 species in 17 genera and 6 families.

Forcipulatida-These asteroids are distinguished by their forcipulate pedicellariae, which are generally quite conspicuous on the body surface. The Forcipulatida contains about 300 species in 68 genera and 6 families.

Notomyotida-These are deep-sea dwelling asteroids having flexible arms with characteristic longitudinal muscle bands along the inner dorso-lateral surface. The Notomyotida contains about 75 species in 12 genera and 1 family.

Paxillosida-These are considered to be somewhat infaunal as they can bury themselves partially under sandy sediments. They are characterised by some plesiomorphic morphological features such as pointed, unsuckered tubefeet. The Paxillosida contains about 255 species in 46 genera and 5 families.

Spinulosida-These asteroids have a relatively delicate skeletal arrangement and completely lack pedicellariae. No fossil spinulosids have been found. The Spinulosida contains about 120 species in 9 genera and 1 family.

Valvatida-These asteroids are quite diverse, but are often characterised by their conspicuous marginal ossicles. The definition of this group, has been highly variable and the ordinal definition of many families has been controversial . The Valvatida contains about 695 species in 165 genera and 14 families.

Velatida-These asteroids typically have thick bodies with large discs and interradial depressions. Classified by Blake (1987), new molecular evidence now suggests a relationship between some velatid and valvatid families. The Velatida contains about 200 species in 25 genera and 5 families.

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Author: Elizabeth Sweet
Last updated: 22nd November 2005
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