Starfish: External Characteristics

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.