BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.8. Kingdom Animalia (animals)

2.8.8. Phylum Echinodermata (echinoderms)

Over 6,000 species of Echinoderms are known. The word "echinodermata" literally means "spiny Skin"; most echinoderms have hard body surface projections that are either spiny or wart-like. All echinoderms are marine animals and lead a predominantly benthic lifestyle, inhabiting coastal zones and shallow seas. Adult forms are characterized by pentamerous Symmetry (a form of radial symmetry), although current evidence suggests their ancestors were bilaterally symmetrical. The Classification and KEY FEATURES OF echinoderms are summarized in Table 2.19, with specific Examples shown in Fig. 2.64.

Class="center">Table 2.19. Classification and diagnostic Features of the Phylum Echinodermata (echinoderms)

Phylum Echinodermata

Diagnostic features

Triploblastic coelomate animals

Exclusively marine

Adults exhibit pentamerous (five-rayed) radial symmetry*

Tube feet used for locomotion*

Calcareous exoskeleton

No distinct HEAD. The Mouth is usually located on the lower (oral) surface, and the anus on the upper (aboral) surface

Class Stelleroidea (starfish / sea stars)

Class Echinoidea (sea urchins)

Flattened, star-shaped body

No sharp demarcation between the central disc and the arms

Body wall contains scattered calcareous plates; spines are movable

Spines present

EXAMPLE: Asterias

Globular shape

Arms absent

Body enclosed in a rigid test formed of tightly fused calcareous plates; capable of covering relatively large distances

EXAMPLE: Echinocardium

* Diagnostic feature

Fig. 2.64. A. The common sea star Asterias. One of the arms is turned over to reveal its underside. Insets show enlarged details. The mouth is located centrally on the lower surface, known as the oral surface. The upper surface of the sea star is called the aboral surface. B. Echinocardium, a Heart urchin.



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