ECHINODERMATA:TYPE STUDY OF STARFISH (ASTERIAS)

Submitted by:Dr. Sakshi VermaAssistant ProfessorZoology Department

ECHINODERMATA

Echinodermata means spiny skinned organisms (Gr. Echinos = spine; derma = skin).

Brittle star

Starfish

Sea cucumber

Antedon

• Exclusively marine and largely bottom dwellers.

• Unsegmented and possess various shapes such asstar-like, cylindrical, disc-like or flower-like.

• Triploblastic.

• Coelomate (enterocoelous)

General Characters:

Difference between Schizocoelom and enterocoelom

• Adults are radially symmetrical, generallypentamerous (various parts are arranged in fives ormultiples of five around the oral-aboral axis).

• Radial symmtery (similar parts are arranged aroundthe central axis in circular manner).

• Larvae are bilaterally symmetrical (Only one planedivides the organism into 2 equal halves).

• Endoskeleton of calcareous plates called ossicles,

embedded in the skin.

• The ossicles usually occur in definite pattern and often bear

projecting spines or tubercles that give the bdy surface a spiny

or warty appearance.

• Water vascular system (derived from larval coelom) is a

unique water filled ambulacral system, helps in feeding or

locomotion.

• Digestive system is usually complete and simple.

• Open type of Circulatory system present, which is reduced

and called as haemal system with no respiratory pigment.

There is no heart or pumping vessel.

• Respiration occurs through tube feet, papulae,

respiratory trees or peristomial gills in different

classes.

• No excretory organ, ammonia is lost through thin

areas of body such as tubefeet, papulae, respiratory

tree etc.

• Nervous system without brain and with a

circumoral nerve ring and radial nerve cords.

• Usually dioecious, gonads large and single or

multiple, opens directly to the exterior by special

ducts.

• External fertilization, indirect development through

a ciliated free swimming larva.

Asterias or Starfish or Sea Star

Systematic position:

Phylum – Echinodermata

Subphylum – Asterozoa

Class – Ateroidea

Order – Forcipulata

Family – Asteridae

Genus - Asterias

Habitat

• Marine, found in almost all seas from tide-marks to deep waters,

preferring rocky areas where locomotion and concealment is easier.

Habits

• Carnivorous (crustaceans, polychaetes and molluscs).

• Nocturnal and walk slowly with the help of their characteristic podia or

tube feet.

• Remarkable power of autotomy and regeneration.

Habits and Habitat

• Central disc with five radiating arms.

• Some genera have more than 5 arms e.g. Solaster (7-14) and Heliaster (>40)

• Size – 10-20 cm in diameter.

• Colour – variable including shades of brown, yellow, orange, pink and purple

•Pentagonal aperture.

•On oral surface

•Surrounded by perioral membrane or peristome

Mouth (Actinostome)

•5; narrow channels; each radiate from angle of mouth and extend in each arm upto the tip.Ambulacral groove

•On oral surface

•Tubular processes arranged in 2 double rows, each on either side of ambulacral groove.

Tube feet

•Outgrowths from ossicles and present on both oral and aboralsurfaces.Spines

•Occur in association with spines.

•Microscopic pincer-like bodies.

•Consists of a flexible stalk and 3 calcareous plates (Forcipulate)Pedicellariae

Morphology of Asterias

Cluster of pedicellariae, papulae and tubercle

A) Straight type pedicellariae; B) Crossed typepedicellariae

• Flat, circular area present on Aboral surface

•Present in an interradius between two arms.

•Bivium and trivium

Madreporite

•Minute circular aperture, situated close to the centre on aboralsurface.Anus

•Minute, finger like hollow retractile processes.

•Projecting through minute dermal pores of integument.

•Respiratory and excretory organ.

Dermal branchiae or papulae

•Each arm has Terminal tentacle (small, median, non retractile and hollow projection) – tactile and olfactory organ.

•Photosensitive eye spot at the base of each terminal tentacle.Sense organs

Morphology of Asterias

Body wall

DIGESTIVE SYSTEM

• In Asterias, the alimentary canal is tubular, straight, short

and extends vertically along the oral-aboral axis in the central

disc. It comprises of the following parts:

1. Mouth: Anterior most aperture of alimentary canal and it is

situated in the centre of the peristomial membrane of the oral

surface. It is provided with a sphincter muscle and radial fibres

and is capable of great expansion and retraction.

2. Oesophagus: Very short, wide and vertical tube.

3. Stomach: Broad sac and fills the interior of the disc. It is

typically divided by a horizontal constriction into a voluminous

oral part, the cardiac stomach and a flattened aboral part, the

pyloric stomach. The cardiac stomach has a muscular, highly

folded wall bulged out to form five lobes, one opposite each

arm.

During the feeding process, the cardiac stomach can be

everted through the mouth by the contraction of muscles of

body wall.

The pyloric stomach is much smaller, flat and pentagonal

sac. It communicates with the intestine.

4. Intestine: Short, narrow, five sided tube that runs

straight upward to open out at the anus.

It gives off two or three little hollow diverticula called

intestinal or rectal caeca.

5. Anus: The anus is situated eccentrically on the aboral

side of central disc.

Digestive Glands:• Ten, long brownish or greenish glandular appendages

variously called pyloric caeca, digestive glands, branchialcaeca, hepatic caeca, etc. are attached to the pyloricstomach.

• There are two pyloric caeca in each arm, each suspendedfrom the aboral wall of the arm by two longitudinalmesenteries that enclose between them a coelomic spacecontinuous at its central end with the general coelom of thedisc.

• The pyloric caeca are lined by ciliated columnar epitheliumhaving glandular and storage cells- secrete a digestive juicesimilar to pancreatic juice of vertebrates.

• The cilia in the pyloric caeca beat in two directions causingopposite currents.

WATER VASCULAR OR AMBULACRAL SYSTEM

6. Radial canal

A)Madreporite seen from outside, B) V.S. through madreporite

Ambulacral System

HAEMAL OR CIRCULATORY SYSTEM

• Open type of circulatory system.

• Largely enclosed in the coelomic sinuses forming the

perihaemal system.

• Comprised of:

1. Axial Gland

2. Aboral Haemal Ring with interradial branches.

3. Tributary Haemal Channels

4. Oral Haemal Ring

RESPIRATORY SYSTEM

Papulae or dermal branchiae and tube feet act as

respiratory organs.

• Papulae: are simple, contractile, transparent, hollow

outgrowths of body wall on the aboral surface having ciliated

epithelium at outer and inner surfaces.

• They are derived from coelom and their lumen remains in

direct communication with coelom.

• An exchange of O2 and CO2 takes place between sea water

and body fluid.

EXCRETORY SYSTEM

• Special excretory organs are lacking inn Asterias.

• The nitrogenous waster materials, mostly ammonia, pass

from the various tissues into the coelomic fluid and then

diffuse into the surrounding water through thin walls of the

rectal caeca, tube feet and dermal branchiae.

• The coelomocytes have significant role in the excretion of

excretory wastes from the coelom.

HAEMAL OR CIRCULATORY SYSTEM

Eyes Neurosensory CellsTerminal Tentacles

REPRODUCTIVE SYSTEM

Unisexual.The males and females are indistinguishable externally.

LIFE HISTORY AND DEVELOPMENT

External fertilization.

Zygote Cleavage: Holoblastic, indeterminate and radial

Larval Forms: A) Dipleurula, B) Formation of bipinnaria

Larval Forms: C) Formation of bipinnaria, D) Bipinnaria, E) Brachiolaria

Development and life history of Asterias

Regeneration and Autotomy of Asterias

Remarkable power of regeneration is observed in

Asterias. It is capable to regenerate its any lost part

of body at any time.

Moreover, if an arm is injured or held up, Asterias

usually casts it off near the base at the fourth or fifth

ambulacral ossicle. This is called autotomy.

• When all the five arms are cut off, regeneration

can still occur but only if the starfish is fed properly.

• An isolated arm cannot develop the central disc

and other arms in Asterias.

• In some species, such as Linckia, a single isolated

arm can grow into a new starfish . Such species cut

off their arms as a method of reproduction.

Regeneration and Autotomy of Asterias

REFERENCES:

1) P.S. Dhami and J.K. Dhami. A text book of Zoology, Nonchordates, Vol. I PradeepPublications.

2) K.L. Kotpal. Modern Text Book of Zoology, Invertebrates, 11th edition. RastogiPublications.

3) Web reference:http://www.biologydiscussion.com/invertebrate-zoology/starfish/asterias-starfish-history-

habitat-and-development/27860