Characteristics Of Echinoderms

Echinoderms are characterized by radial symmetry, several arms (5 or more, mostly grouped 2 left - 1 middle - 2 right) radiating from a central body (= pentamerous). The body actually consists of five equal segments, each containing a duplicate set of various internal organs. They have no heart, brain, nor eyes, but some brittle stars seem to have light sensitive parts on their arms. Their mouth is situated on the underside and their anus on top (except feather stars, sea cucumbers and some urchins).

Echinoderms have tentacle-like structures called tube feet with suction pads situated at their extremities. These tube feet are hydraulically controlled by a remarkable vascular system. This system supplies water through canals of small muscular tubes to the tube feet (= ambulacral feet). As the tube feet press against a moving object, water is withdrawn from them, resulting in a suction effect. When water returns to the canals, suction is released. The resulting locomotion is generally very slow.

Characteristics

• All marine

• Known as spiny-skinned animals

• Endoskeleton known as the test is made of calcium plates or ossicles with protruding spines

• Includes sea stars, brittle stars, sand dollars, sea urchins, & sea cucumbers

• Undergo metamorphosis from bilateral, free-swimming larva to sessile or sedentary adult

• Larval stage known as dipleurula or bipinnaria

• Adults have pentaradial ( 5 part) symmetry

• Lack segmentation or metamerism

• Coelomate

• Breathe through skin gills as adults

• Capable of extensive regeneration

• Ventral (lower) surface called the oral surface & where mouth is located

• Dorsal (upper) surface known as aboral surface & where anus is located

• Have a nervous system but no head or brain in adults

• No circulatory, respiratory, or excretory systems

• Have a network of water-filled canals called the water vascular system to help move & feed

• Tube feet on the underside of arms help in moving & feeding

• One-way digestive system consists of mouth with oral spines, gut, & anus

• Deuterostomes (blastopore becomes the anus)

• Separate sexes

• Reproduce sexually & asexually

• Includes 5 classes:
  * Crinoidea - sea lilies & feather stars
  * Asteriodea - starfish
  * Ophiuroidea - basket stars & brittle stars
  * Echinoidea - sea urchins & sand dollars
  * Holothuroidea - sea cucumbers

Ecology And Range Of Echinoderms Echinoderms are exclusively marine. They occur in various habitats from the intertidal zone down to the bottom of the deep sea trenches and from sand to rubble to coral reefs and in cold and tropical seas. Behaviour Of Echinoderms
Some echinoderms are carnivorous (for example starfish) others are detritus foragers (for example some sea cucumbers) or planktonic feeders (for example basket stars).

Reproduction is carried out by the release of sperm and eggs into the water. Most species produce pelagic (= free floating) planktonic larvae which feed on plankton. These larvae are bilaterally symmetrical, unlike their parents (illustration of a larvae of a sea star below). When they settle to the bottom they change to the typical echinoderm features.

Echinoderms can regenerate missing limbs, arms, spines - even intestines (for example sea cucumbers). Some brittle stars and sea stars can reproduce asexually by breaking a ray or arm or by deliberately splitting the body in half. Each half then becomes a whole new animal.

Echinoderms are protected through their spiny skins and spines. But they are still preyed upon by shells (like the triton shell), some fish (like the trigger fish), crabs and shrimps and by other echinoderms like starfish which are carnivorous. Many echinoderms only show themselves at night (= nocturnal), therefore reducing the threat from the day time predators.

Echinoderms serve as hosts to a large variety of symbiotic organisms including shrimps, crabs, worms, snails and even fishes.