Phylum EchinodermataSea Star, Sea Urchin, Sea Cucumber

General Characteristics

• Spiny skinned• pentaradial symmetry = as adults have five

similar parts arranged in circle around a central point

• exclusively marine, adapted to a bottom-dwelling existence

• examples include sea stars (‘starfish’), sea urchins, sand dollars, sea cucumbers

• most species reproduce sexually, have separate sexes, and have external fertilization. Note that the larvae of echinoderms are ciliated and bilaterally symmetric .

• Many echinoderms are recognized by their pentaradial (five-fold) symmetry. It is important to understand that the radial symmetry of echinoderms is superficial and almost certainly secondarily derived. This means that the ancestor of echinoderms (the ancestor it shares in common with all other bilaterians) was likely bilaterally symmetric. If we look carefully at a typical echinoderm like the sea star we will note that the five part organization can be divided into mirror-image right and left halves - the hallmark of bilateral symmetry.

• An easy way to see this is to find the madreporite and imagine a line connecting it to the tip of the arm opposite the madreporite. This line will divide the animal into right and left halves.

Muscular/ Skeletal System• Mouth on the bottom

side of organism (oral surface). Other side= aboral surface

• have an internal skeleton (endoskeleton) composed of interlocking calcium carbonate plates

Water Vascular System

• Contains a water vascular system• Water enters body through a sieve plate

(=madreporite) The water is transport through the ring canal and the radial canal to power the tube feet

• have tube feet, which are versatile structures used for locomotion, gas exchange, feeding, attachment, and sensory reception!

• Tube feet:..\..\..\Lesson Videos\Biology 11\Arthropods and Echinoderms\Starfish or Sea Star (Asteroidea).flv

Digestion• Use tube feet to open

bivalves, to capture preys• Sea stars and sea urchins

contain pedicellaria = grasping appendages

(food gathering, cleaning, & defense against predators)• Sea stars can flip its stomach out of the mouth and

digest/ eat the food outside its body. It will flip its stomach back in when finished.

• Herbivores (e.g. sea urchins) have 5-part jaws to scrape algae

• Some are filter feeders (sea lillies, brittle stars)• Some detritus feeders

Respiration• Diffusion through tube feet

Circulatory• Done by digestive gland and fluid in body

(located in pseudocoelem)

Excretory• Solid food waste anus• Nitrogen waste tube feet, gill, all over body

Nervous System• No brain (radial symmetry)• Nerve rings around mouth; eyespot; statocysts

Movement• Use tube feet

Reproduction• Echinoderm sexes are generally separate • Fertilization and development of young are

external, larval stage in life cycle

Class Asteroida (Sea Stars)

• 5-26 arms• eyespots located on tip of each arm• push stomachs out of body through mouth to

devour prey• can regenerate lost or damaged arm, but is

very slow• gills protrude through skin

Crown of thorns starfish

• Reading and species file

• Video:..\..\..\Lesson Videos\Biology 11\Arthropods and Echinoderms\Crown-of-Thorns starfish.flv

Know your anatomy

Worksheet time