echinoderm joanne yip 6s 31. echinoderms a phylum of marine animals invertebrates found at every...

Echinoderm

Joanne Yip

6S

31

Echinoderms

• A phylum of marine animals

• Invertebrates

• found at every ocean depth

• have complete digestive gut

• have a haemal system,

• mesodermal skeleton composed of calcareous plates or ossicles

5 related classes in the phylum Echinodermata :

1. Sea star or starfish (Asteroidea)

2. Brittle stars, basket stars, serpent stars (Ophiuroidea)

3. Sea urchins, heart urchins and sanddollars (Echinoides)

4. Holothurians or sea cucumbers (Holothuroidea)

5. Feather stars and sea lilies (Crinoidea)

Sea Star

http://www.treknature.com/gallery/Africa/Egypt/photo1222.htm

Brittle stars

http://www.treknature.com/gallery/photo80622.htm

Sea urchins

http://www.treknature.com/gallery/photo127421.htm

Sea cucumber

http://www.treknature.com/gallery/photo52531.htm

Feather star

http://www.treknature.com/gallery/photo80741.htm

Features

• radial symmetry

• several arms radiating from a central body

• body consists of five equal segments (each containing a duplicate set of various internal organs )

• have no heart, brain, or eyes

• some have light sensitive parts on their arms (brittle stars)

• 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.

• Gaseous exchange also takes place through tube feet

• This system supplies water through canals of small muscular tubes to the tube feet, water is withdrawn from them, resulting in a suction effect.

• When water returns to the canals, suction is released. The resulting locomotion is ge

nerally very slow.

Reproduction

(i) Sexual reproduction • become sexually mature after

approximately two to three years

• eggs and sperm cells are released into open water

• release of sperm and eggs is coordinated temporally in some species

• Internal fertilization has currently been observed in some species of sea star, brittle stars and a deep water sea cucumber.

Asexual reproduction

• Sea cucumbers often discharge parts of their internal organs if they perceive danger

• discharged organs and tissues are quickly regenerated

• Sea urchins are constantly losing their spines through damage ( all parts are replaceable)

• Some sea stars are capable of regenerating lost arms

• lost arms have been observed to regenerate a second complete sea star

• Some sea star populations can reproduce entirely asexually purely by the shedding of arms for long periods of time.

Sun flower sea star regenerates its arms

Evolution of echinoderms

Case1• evolved from animals with bilateral symme

try

• that radial symmetry arose in a free-moving echinoderm ancestor

• sessile groups were derived several times independently from free-moving ancestors.

Case2(traditional view)

• the first echinoderms were sessile, became radial as an adaptation to that existence

• gave rise to free-moving groups

• Attachment to a substratum would have selected for radial symmetry

• may have marked the origin of the Class Crinoidea.

Members of Crinoidea,

• were primitively attached to a substratum by an aboral stalk.

• An ancestor that became free-moving might have given rise to Asteroidea, Ophiuroidea,Echinoides and Holothuroidea

THE END