University of Lille 1 - Faculty of biology

Animal Biology

Sea star dissection

Morphology

The Asteria rubens sea star

is an Echinoderm

belonging to the class of Asteroidea.

It lives on costs

we may find it in the intertidal zone

at low tide.

It feeds chiefly on mussels

fixed on rocks.

The sea star is part of the foreshore food web

and can fall prey to other animals.

The body of the sea star is flattened,

in a shape of a star with 5 arms.

The arms merge at their base

and are attached to the flat disk.

The arms are called radius

The zones between the arms are called interradius.

This organisation is representative of the pentaradial symmetry,

characteristic of the adult echinoderms.

The sea star has two faces:

the upper face,

or aboral surface,

is convex and pigmented.

The bottom side

or oral surface,

is flattened, and clearer,

in contact with the substrate.

This surface includes the mouth,

located in the middle of the flat disk.

On the aboral surface,

at the interradius level,

we can see a whitish limestone plate,

the madreporic plate.

There are bony spines convering the body,

organized in rows.

On the aboral surface,

there is a row in the middle of each arm

and two or three lateral rows.

By turning the sea star over,

we can see a supramarginal row

and an inframarginal row

Two rows of prikly adambulacra

delimit a central furrow,

the ambulacral groove,

Every ambulacral groove has 4 rows of fleshy and movable tubes

tube feet or podia,

ended by a sucker.

At tip of each arms we can see eyespots,

sensory organs red in coulour

sensitives to the light.

Integument

In the sea star,

the body and the spines are covered

by an ciliated epidermis

The dermis,

situated beneath the epidermis,

has calcareous plates,

or ossicles

in which the spines are articulated.

The locations of ossicles

are visibles on the surface of the animal.

The integument has specialized structures,

pedicellariae,

put in the shape of a crown at the base of the spines.

In Asteroidae,

they have a form of a pincer with two jaws.

Pedicellariae are used

to clean the integument

and the defense of the animal

Organs used for the breathing,

gills or papules,

are associated with the integument.

they are between the ossicles of the skeleton.

The presence of an epithelium

ciliated on the inner side of the integument

enables the circulation of the liquid

inside the body cavity,

the coelom.

Echinoderms have a calcareous internal skeleton,

formed by ossicles.

the cross section of the arm enables us

to observe the organisation of the dermal plates.

We can see:

a dorsal ossicle

two supramarginal plates

and two inframarginal plates each having spines,

two adambulacra plates

and two ambulacral plates.

Between the ambulacral plates,

there are pores that allows the passage of the tube feet.

General anatomy

To observe the internal organisation of the sea star,

we have to cut the sea star with scissors.

Incise the integument

following the outline

to separate the upper half

from the lower half of the animal.

Identify the madreporic plate

and avoid it when you incise.

Lift the integument of the aboral surface

starting from the end of the arms

and cut carefully the underlying organs

ending by the flat disk.

The rest of the dissection will be done in the water.

In the coelom we can see gonads

and a part of the digestive system.

The digestive system

The digestive system includes the mouth,

the stomach divided in two superimposed chambers,

the pyloric ceca and the rectal ceca.

It ends by a microscopic anus,

located in the aboral surface.

The observation of the digestve system

will be done using the aboral portion.

In the central part of the disk,

we notice two rectal ceca.

Below we notice the upper chamber of the stomach,

the pyloric portion with a pentagonal shape.

5 canals start from this chamber

that extend themselves in the arms of the star.

Each pyloric canal is connected to two pyloric ceca

that take up a large part of the arms.

The pyloric ceca,

composed by dozens of diverticula on dead end

play a digestive key role

and are used to the storage of nutrients.

To observe the rest of the digestive system

we have to remove the rectal ceca,

pyloric ceca

and the pyloric stomach.

The ventral chamber or cardiac stomach,

has a shape of five lobes rose.

For the ingestion of preys,

this part of the stomach is everted.

After the external digestion,

the stomach gets back to its position

thanks to the contraction of five pairs of retractor muscles

put in the ambulacral plates.

To see the mouth,

we have to detach the retractor muscles

and remove the portion of cardiac stomach.

The mouth,

located in the middle of the flat disk,

surrounded by a membrane,

the peristomium.

Reproductive system

In Asteroidea

the sexes are separated

but there is not a sexual dimorphism.

Gonads are located in the coelom.

To observe the gonads

we have to remove the pyloric ceca,

and the upper chamber of the stomach.

Every gonad,

with a granular apsect,

is divided in deux lobes

that all extend in two adjacent arms.

The size of gonads is variable,

it depends on the state of sexual maturation of the animal.

Females and males produce gametes

through genital pores located in the interradius

at the junction with the flat disk.

The fecondation is extern.

Aquifer system

One of the characteristic of Echinoderms

is the existence of an aquifer system.

It is involve in the locomotion and the food intake.

This system have a network of conduits

filled with a liquid

whose composition is close to the sea water

The aquifer system have a madreporic plate,

the stone canal,

the ambulacral ring,

five radial canals,

and many tube feet or podia.

To observe the different parts of the aquifer system

we have to open the sea star

and remove the digestive

and reproductive system.

The madreporic plate, or madreporite,

is visible on the aboral surface of the animal.

This calcareous plate is riddle with holes

"aquifer pores"

that permit the sea water to enter in the system.

The madreporite is in connection with the madreporic canal

or stone canal.

This canal

sostained by superimposed calcareous rings,

go down toward the oral surface of the animal.

The stone canal

rejoin the ambulacral ring surrounding the mouth.

This ring is hidden

with calcareous plates of the skeleton.

On the circumference of the ambulacral ring of the star

in the interradial position,

we notice 9 vesicles with an immune function:

Tiedemann bodies.

From the aquifer ring goes in each arm

an ambulacral canal or radial canal,

located between the epidermis and the ambulacral plate.

From the radial canal goes many lateral canals.

Those are connected to the tube feet or podia.

The injection of a dye into ambulacral canal of an arm

enables us to hightlight

a large part of the aquifer system.

Podia have three parts:

ampulla or bulbous ampulla

a muscular and elongated foot

and a sucker at the end.

The podia go through the skeleton at pores level

located between the two adjacent ambucral plates.

To observe those pores

we have to scratch the polian vesicules

on a part of the arm.

The contraction of the polian vesicles

allows the turgescence of the tube feet

and the the monitoring of the sucker 
for the locomotion of the animal

The podia within the ambulacral groove

are protected by rows of movable ambulacral spines

and by many pedicellariae.

Radial canals

end with sensory tentacles

close to the eyespot.

Pedagogical design: Pierre-Eric Sautière Jacopo Viziolo

Technical production: Bernard Mikolajczyk

Designs: Jacopo Vizioli

Computer graphics: 3D MEDICUS

Voice over: Alain Nempont

Music; Ambient slamming bass jam, Jay Berlinsky

Gratitude: Sébastien Lefebvre, Michel Priem

Gratitude: Patrick Flammang, Michel Jangoux

SEEM; Education Service and Multimedia