ECHINODERMS FOR DUMMIESCOLWYN SLEEP

ECHINODERM CLASSIFICATION AND EXAMPLES

What exactly is an Echinoderm? Echinoderms (or “Echinodermata”) are a group of animals which exist only in a marine (ocean)

environment. Their name comes from the Greek word for "spiny skin". They inhabit a diverse range of marine habitats and are found

on the sea floor from the intertidal zone to great ocean depths. In the following slides we will explore echinoderms and look into their body

systems and structures more closely.

A FEW EXAMPLES OF ECHINODERMS

Probably one of the best known echinoderms is the easily recognized sea star. The

echinoderm phylum contains many more species, in fact there are around 7000 known

echinoderms. Later, we will investigate the sea star and it’s classification as an

echinoderm in greater detail.

Sea star Kingdom: Animalia

Phylum: EchinodermataClass: Asteroidea

Feather Star Kingdom: Animalia

Phylum: EchinodermataSubphylum: Crinozoa

Class: Crinoidea

This lesser-known star, the feather star gains its name from its featherlike arms, which it uses to swim through the water. Like all echinoderms, they are “Deuterostomes.” This means that during their embryonic

development the first embryonic opening becomes the anus and the second becomes the mouth. Because of this, biologists believe

that echinoderms are more evolutionarily advanced than some of the other animals.

A FEW MORE…Sea cucumber

Kingdom: AnimaliaPhylum: EchinodermataClass: Holothuroidea

Sea Urchin Kingdom: Animalia

Phylum: EchinodermataClass: Echinoidea

Like other echinoderms, the sea urchin has radial symmetry. Its pointed spines offer

protection, and in some cases are poisonous.

Related to Jabba the Hutt in appearance only, the sea cucumber is actually another example of an echinoderm. Echinoderms have no cephalization, (this means their sensory organs are not located at their “head”. They also have a nerve net for

controlling motor and sensory functions. These evolutionary traits are considered

“less advanced” and can make echinoderms difficult to classify.

THE SEA STAR

Lets look into everyones favourite echinoderm the “sea star” in greater detail…

BODY SYMMETRYEchinoderms have radial symmetry, meaning the basic body plan of

these organisms can be divided into similar halves across a plane at any angle along a central axis. Radial symmetry would usually be considered a more primitive trait than bilateral symmetry however, in their larval

form echinoderms are free swimming bilaterally symmetrical organisms. It is this trait that indicates echinoderms are more complex than other

radially symmetrical animals.

NERVOUS SYSTEM

The sea star nervous system is very simple. They have no brain and no ganglia to coordinate movement. Their nervous system is characterized by a nerve ring that surrounds the mouth. A

radial nerve branches off of the nerve ring and extends to each arm. To detect light, they have "eyespots" at the tip of each arm. The eyespot contains light sensitive pigments that allow them to

detect shadows and changes in the brightness of light.

DIGESTIVE SYSTEM

The sea star’s mouth is located on the under side of the sea star. Food enters the mouth and then the first stomach (cardiac). Then it is passed onto the secondary stomach (pyloric) and into the digestive glands that are located within the arms of the starfish. The digestive glands secrete enzymes and absorb nutrients. An intestine

runs from the secondary stomach to the anus which is located in the central part of the body. Some species of starfish eat their prey whole and begin digesting it before sending it to the digestive glands in the arms of the starfish. Others put the first stomach outside of their body (almost like it is turned inside out)

where it can eat food and begin the digestion process. It then passes the food to the secondary stomach, which always stays inside.

Using its water vascular system, the starfish can evert its stomach into clams and mussels; forcing them open so the starfish can eat the animal inside the shell. This process allows the starfish to eat animals including clams,

mollusks, small fish and oysters that are much too large to fit into the starfish's tiny mouth.

RESPIRIATORY SYSTEM

Respiration occurs in two locations on a sea star although they have no specific respiratory organs. The first location is through the thin skin on the

ends of the tube feet. The second location is in the dermal gills or papulae. These gills are lined with cilia

that move to produce a current. The sea current brings new fresh oxygen rich water and removes

the sea star’s excreted carbon dioxide.

MOVEMENT

For movement, sea stars have a water-vascular system and tube feet. The underside of a sea star is covered with hundreds of tube feet. It uses these to move around, for attaching to rocks, and for

holding on to prey. To move, each tube foot swings like a leg, lifting up, swinging forward, and planting itself on the ground. At the tip of each tube foot is a suction cup. The bulb at the top of the tube foot is called the ampulla. When contracted, the ampulla squeezes water down into the tube foot, which

extends. To retract the tube foot, the ampulla relaxes. The extension and retraction of the tube feet are by water pressure, muscles are used to bend the tube feet. The tube feet and the ampullae are part of

the sea star’s water vascular system.

Tube Feet-side view

REPRODUCTIVE SYSTEM

Most species of sea stars have male and female individuals. Other species are hermaphrodites and produce eggs and sperm at the same time. Each arm contains two gonads that release gametes through openings called gonoducts. Fertilization is usually external, the buoyant eggs and sperm are released into the water and the embryos and larvae live as part of the plankton.Most starfish embryos hatch at the blastula stage. The first entrance to form during the embolic development is called the blastopore and later develops into the anus. A band of cilia develops on the exterior and extends around the surface eventually onto two developing arm-like outgrowths. At this stage the larva is a free swimming bilaterally symmetrical organism.Next, metamorphosis occurs with a rearrangement of body tissues. The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. The mouth and anus move to new positions. The transformation changes the sea star from it’s bilaterally symmetrical laval form into its radially symmetrical adult form.Some species of sea stars can regenerate lost arms and can regrow an entire new limb given time. A few can regrow a complete new disc from a single arm. Regrowth can take several months or years.

FUN FACTS

Sea stars can be found in all oceans of the world and are usually located in the shallow water.

Commonly called "starfish", sea stars are not related to fish at all.

Some sea stars can live up to 35 years old

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