STARFISH
SYSTEMATIC POSITION
Kingdom: Animalia
Phylum:
Echinodermata
Class: Asteroidae
Order: Forcipulata
Family: Asteriidae
Genus: Asterias
Species: rubens
HABIT & HABITAT:
1. Asterias is exclusively marine in nature
2. It is mostly found on sea costs and in shore
waters
3. It generally prefers rocky areas where
locomotion and concealment is easier
4. It is bottom dwelling benthoic organism
5. It clings to submerged objects by firmly
attaching to them.
6. It is solitary in nature in normal conditions
7. Asterias is strongly positive towards light
whereas other genus are negative towards light
8. It is carnivorous and voracious feeder
9. It is nocturnal in nature
10. It shows remarkable power of autotomy and
regeneration
EXTERNAL
CHARACTERISTICS:
A. SIZE & SHAPE:
1. Body is star shaped, dorsoventrally flattened,
radially symmetrical and has pentamerous arrangement.
2. Pentagonal central disc is present through
which 5 elongated, tapering symmetrically arranged projections called rays or
arms are present.
3. Generally 5 arms are present (exception sun
star where 11-13 arms are present; for some genus number of arms may vary from
7 to 40)
4. Length of arm is 2 to 3 times the diameter of
central disc.
5. The axis of arms are called as radii and
regions of central disc between the arms are called as inter radii
6. Body has oral or actinal surface where mouth
is present
7. Oral surface is present downwards attached to
substratum
8. The upper surface is called as aboral or
abactinal surface
9. Aboral surface is convex and is covered with
spines of various lengths.
10. At the center of
aboral surface minute opening called anus is present
B. COLOUR:
Asterias shows many colour variations like
yellow, brown, orange etc.
C. STRUCTURES ON ORAL SURFACE:
1. MOUTH:
i. It is also called as actinosome
ii. It is a circular aperture situated at the
center of oral surface of central disc.
iii. It is surrounded by soft membrane called as peristomal
membrane or peristome.
iv. Peristome is guarded by 5 groups of oral
spines or mouth papillae
2. AMBURACERAL GROOVE:
i. These groove originate from 5 corners of mouth
ii. Each groove is variable in width and runs
along middle of the oral surface of each arm up to its tip
iii. Each groove shows two rows of tube feet provided
with terminal discs
3. AMBURACERAL SPINES:
i. Each ambulaceral groove is bordered and
guarded from lateral sides by 2 to 3 rows of movable calcareous ambulaceral
spines.
ii. These spines can closes over groove
iii. Near mouth these spines become larger and
stouter and grouped together to form mouth papillae
iv. On outer sides three rows of immovable spines
are present.
v. Similarly, on the borders of arm marginal
spines are present which separates oral and aboral sides of the organism.
4. TUBE FEET OR PODIA:
i. Each ambulaceral groove contains double rows
of soft, thin walled, extensile, tubular structures called as tube feet.
ii. Tube feet ends in a sucker and the sucker
works as a suction cup for firm attachment on the surface to which it is
applied
iii. It is multipurpose organ but mainly are used
for locomotion and foot capturing
iv. These are also used for respiration and
adherence.
v. These are also sensory in function.
5. EYES & TENTACLES:
i. At end of each ambulaceral groove, these are
small light sensitive, bright reddish pigment spot called as eye in starfish.
ii. Each eye is made up of no. of ocelli.
iii. Above aye projects a median non-retractile
process called as tentacle.
iv. Thus, there are 5 unpaired eyes and tentacles.
v. Tentacles are tactile and olfactory in
function.
D. STRUCTURES ON ABORAL SURFACE:
1. SPINES:
i. These are short, stout, blunt & immovable
calcareous structures covering entire aboral surface.
ii. These are outgrowths of calcareous plates
embedded in body wall.
iii. Spines are arranged in irregular rows,
parallel with the axes of arms.
2. DERMAL BRANCHIAE ORPAPULAE:
i. Large number of minute dermal pores are
present between ossicles of integument
ii. Each dermal pore projects out a very small,
soft delicate, hollow finger like membranous retractile process called as
dermal branchia or gills or dermal papulae.
iii. These processes can be completely retracted in
body.
iv. Each papula is hollow evagination of the body
wall containing extension of coelom.
v. As wall of papula is thin, it is respiratory
in function through thin wall; oxygen diffuses to coelomic fluid and CO2diffuses
out.
vi. Amoebocyte cells pass out through this wall
along with waste products.
vii. Thus, also performing excretory function.
3. ANUS:
i. It is small aperture, hardly visible to naked
eyes.
ii. It lies nearly in the center of aboral surface
being slightly displaced towards the inter radius next to that occupied by
madreporite.
iii. Echinoderms are the only animals having anus
dorsal in position.
4. MADREPORITE:
i. It is flat, circular, small but conspicuous
button like structure.
ii. It is situated at aboral surface between two
arms.
iii. The two rays between which madreporite is
placed are called as bivium and remaining 3 rays are called as trivium.
iv. Madreporite is sieve like porous plate and
leads to the stone canal of water vascular system.
v. Generally single madreporite is present but
sometimes more than one madreporite may also be present due to increased number
of arms.
vi. Asymmetrical position of madreporite converts
radial symmetry of starfish to bilateral.
PEDICELLARIAE:
i. These are modified spines.
ii. Occur in space between spines or in clumps
around the bases of spines all over the body.
iii. These are microscopic jaw like bodies
Structure Of
Pedicellariae:
i. Pedicellariae: are stalked i.e. with peduncle
in Asterias
ii. Each pedicellaria consists of short flexible
and fleshy stalk
iii. Stalk bears three calcareous plates or
ossicles.
iv. One plate is basilar plate while other two are
jaws or valves like.
v. Jaws articulate with basilar plates and are
serrated along their opposite edges
vi. These are covered with epidermis, which is
richly supplied with sensory and glandular cells.
Types Of
Pedicellariae:
There are two types of pedicellariae found in Asterias viz.
a) Forceps or straight type
b) Scissors or crossed
type.
Forceps or Straight
Type:
i. It is simple in which two jaws are more or
less straight and are attached basely to basal plate.
ii. When pedicellariae is closed, jaws remain
parallel and meet throughout their length like forceps.
iii. The jaws can be opened or closed by muscles
iv. Jaws are operated by two pair of muscles
· Abductor muscles- for opening
· Adductor muscles – for closing
Scissors or Crossed
Type:
i. These are small and are arranged in rings around
the white spines on aboral surface.
ii. In this, the basal ends of the two jaws are
curved and cross each other like mandibles of cross bill.
iii. Basal pieces are enclosed between crossed
portions.
iv. One pair of abductor muscles 3 pairs of
adductor muscles effects movement.
v. The abductor muscles originate on opposite
ends of basal ossicles and insert on neighboring crossed parts of jaws.
vi. An elastic ligament is present in the stalk,
which bifurcates, for attachment to outer surface of the basal end of each jaw.
Functions of
Pedicellariae:
i. Protects delicate skin, gills or papulae
ii. Keeps body surface free from debris and
foreign organisms
iii. These also function as defensive and offensive
organs
iv. In some starfishes, these are used to capture
small prey
v. These are also sensitive to contact.
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