Phylum Echinodermata

(Echinoderms)

General characteristics

• Invertebrates• Marine environments• not cephalized, no head• Deuterostromes (blastopore forms an

anus)• Coelmates• embryos have radial cleavage

4 major unique characteristics

• 1- radial symmetry – Often pentaradial symmetry, body parts extend from

the center along five spokes

• 2- endoskeleton made of ossicles- calcium carbonate plates – may be attached to spine– Or spicules that protrude through skin, forms

echinoderm- “spiny skin”

• 3- water-vascular system– Network of water-filled canals

• 4- Tube feet– Small, movable extensions of the water vascular system– Used for movement, feeding, respiration, and excretion

Evolution

• Fossil record dates back to more than 500 million years (the Cambrian period)

• Early echinoderms were sessile• Only 80 species still sessile• Now some can crawl slowly along ocean

bottom• The bilaterally symmetrical larvae

resemble it’s ancestors• Most closely related to chordates

Class Crinoidea

• means “lily-like”• Five arms branch to form many more arms• Mucus covered tube feet filter organisms from

the water• Cilia then transport the organisms to the mouth• Sea lilies

-Sessile adults• Uses a long stalk to attach to rocks or the sea bottom

• Feather stars– Can swim and crawl as adults

• Most species are extinctFeather star – Florometra serratissima

Class Ophiuroidea• Means “snake-tail”• Long, narrow arms allow them to move quicker

than other echinoderms– Tube feet not used for movement

• Basket stars have thin, flexible limbs form coils that look like tentacles

• Parts of brittle stars break off easily– Can regenerate

green brittle star – Ophiarachna incrassata

Basket star – Gorgonocephalus eucnemis

Class Echinoidea• Means “spinelike”• Sea urchins and sand dollars• Test- rigid exoskeleton, encloses internal organs• Sea urchins have an Aristotle's mantle-complex

jaw like mechanism, aids in movement• Spines can contain poison

Purple Sea Urchin – Heliocidaris erythrogramma

Sand Dollar – Echinarachnius exentricus

Class Holothuroidea• Means “water polyp”• Sea cucumbers• Armless• Live on sea bottom, use tube feet to burrow• Ossicles are very small and not connected, which

means their body is soft

California sea cucumber - Parastichopus californicus

Class Asteroidea

• Starfish– Carnivorous (mollusks, worms, other

slow moving animals)– Can slowly pull clam apart• use a cardiac stomach to begin digestion• withdraws the stomach • finishes digestion

Red-knobbed sea star- Protoreaster linckii

Sea Star digestion

Mouth– Cardiac stomach- short esophagus– Pyloric stomach– Digestive glands (in each arm)• secrete enzymes that break down food

– Nutrients go to the coelem– Undigested material expelled through

anus

Sea Star external structure

• The aboral surface (top half) is covered in short spines– rough texture

• Pedicellariae- tiny pincers that surround the short stems– keep the body free of foreign objects,

ex. Algae or other small animals

Sea Star Body Systems

--No circulatory system– No excretory system– No respiratory system

• Fluid in the coelem distributes nutrients and oxygen

• Diffusion through skin gills on tube feet (respiratory gases and waste)– Skin gills- hollow tubes, stretch from

coelem to exterior

Nervous system

• No head= no brain• Nerve ring instead (circles the mouth)• Radial nerve runs from the nerve ring along each

arm• Together coordinate tube feet movement• Nerve net near body surface controls movement of

spines, pedicallariae, and skin gills• Eyespot at the end of each arm responds to light• Several tentacles and tube feet respond to touch• Touch-sensitive and chemical-sensitive cells are

scattered all over the surface of the sea stars body

Reproduction and development

• Most have separate sexes• Each arm contains a pair of ovaries and testes• External fertilization, • Larvae develops bilaterally symmetrical, called bipinnaria• Some retain their larvae in their body or among their

tentacles

• Can regenerate arms, but it takes about a year– Can shed an arm to escape a predator

• If an arm is attached to a portion of the central region, a whole new organism can develop

• Some species reproduce asexually– By splitting it’s body through the central region

Sea StarWater-Vascular System

• Water enters through the sieve-like madreporite on the aboral surface

• Passes through stone canal• Then the ring canal• Then the Radial canals• Contraction of muscles

surrounding ampulla cause the tube feet to

expand and contract• Ends of tube feet act

as suction cups• Allows the sea star to move

Works Cited• Charton, Barbara. "sea star." The Facts On File Dictionary of Marine

Science, New Edition. New York: Facts On File, Inc., 2007. Science Online. Facts On File, Inc.http://www.fofweb.com/activelink2.asp?ItemID=WE40&SID=5&iPin= DMSN1384&SingleRecord=True (accessed April 9, 2009).

• "Echinoderms: Features." Animal Anatomy On File, New Edition. Copyright Diagram Visual Information Ltd., 2003. Science Online. Facts On File, Inc. http://www.fofweb.com/activelink2.asp?ItemID=WE40&SID=5&iPin= A0439&SingleRecord=True (accessed April 9, 2009).

• Edward J. Devillez, "Digestion (invertebrate)", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.194500

• David L. Pawson, Andrew C. Campbell, David L. Pawson, David L. Pawson, Raymond C. Moore, J. John Sepkoski, Jr., "Echinodermata", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.210700