echinodermata: type study of starfish (asterias · echinodermata means spiny skinned organisms (gr....
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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