Sea Star -Exrernal Morphology

ECHINODERMATA GENERAL CHARACTERS AND CLASSIFICATION

 Asterias is a marine and widely distribu­ted member of echinoderm. All the species under this genus are benthic animals, as they inhabit the bottom of the sea. They are quite abundant on various types of sea-bottom but prefers a rugged and rockbound coast, where they can hide very easily and lead a sluggish life. They also live in deep water.

They are carnivorous and predacious animals, preferring mainly bivalves as food. Majority of the forms are photonegative and prefer to live in shaded areas. A few exceptions are the Asterias rubens, A. gibbosa, A. panceri, where positive response to light is observed.

Echinodermata means spiny skinned animals. This name was first proposed by Jacob Kelin (1734) for echi-noids. It was Leuckert (1847) who first established Echinodermata as a distinct group of the animal kingdom.

 

1)   They are exclusively marine.Echinodermata-star fish

2)   They are free living. Some are pelagic. Some are creeping on the sea bottom.

3)   These are all triploblastic and co-elomate animals.

4)   Adults show pentaradial symmetry and larval form show bilateral symmetry.

5)   The size varies from moderate to considerable size.

6)   Shape of the body is spherical or globular or elongated. It shows oral aboral and sides.

7)   Exo and endoskeletons will be present.

8)   Endo-skeleton is mesodermal. It is made by hard polygonal plants.

9)   Body walls include only Epidermis, dermis and peritoneum.

10) Body cavity or coelome is large. It is an entero-coelome. It is lined by         ciliated peritonium.

12)  Respiration is carried on by   Papellae

13)   Excretory organs are absent.

14)   Nervous the system is primitive. Brain is absent.

15)   Special sense organs are poorly developed. Eye spots and statocysts  are present.

16) Reproduction is sexual. Sexes are separate. Sexual dimorphism is absent. Gonads are large.

17. Gametes are liberated into the water. Fertilisation is external.

18) Eggs are homolecithial. Cleavage is a radial and indeterminate type.

19) Life history includes larval stage.

20)   They can show autotomy and regeneration.

 

Star fish or Sea star is common echinoderm in the sea water. Its Zoological name is Asterias rubens. 

Classification

Phylum :             Echinodermata

Sub-Phylum       Asterozoa

Class :                  Stelleroidea

Sub-Class            Asteroidea

Order :                 Forcipulata

Genus                  Asterias

Species                rubens

 

a.      The genus Asterias is represented by 100 species,

b.     Star fish dwells on the bottom of the sea.

c.      They are more common on hard rocky sea bottom,

d.     Star-fish is a carnivorous animal.

e.      It creeps slowly on the sea bottom.

f.       It can bend or twist in many ways.

g.     It has the power of regeneration.

 

 External Features :

The body of Asterias is star shaped and looks like a sea star.

 Shape:

 1)   The body, is compressed on oro-aboral axis.

2)   It shows a central disc From this disc five arms will project.

3)   They are symmetrically arranged.

4)   The arms are board proximalhy and are free at the distal end.

5)   The arms occupy the radial axis.

Size :

 The smallest star fish is one cm in diameter, where as the largest one is 200 cm in diameter.  The average size varies from 10 to 20 cm.

  Colour:

 Star fish show brilliant colours. Yellow - brown or orange colours are common.

Surface of the Body :

The body shows two surfaces.

 

i) Oral

ii) Aboral.

 

1)           The oral surface faces the bottom of the sea.

2)           The centre of the oral surface contains the mouth.

3)           The aboral surface is directed upwards. It is slightly convex.

4)           The oral and aboral surfaces are not dorsal and ventral sides of the body,         but right and left sides of a bilaterally symmetrical animal.

Oral surface or Actinal surface:

 It is the lower surface of the animal. It is directed downwards. It is flat. It shows the following parts.

 1.    Mouth:

It is a round opening. It is present in the center of the oral surface of the central disc. The mouth is also called Actinosome. It is surrounded by a peristomial membrane, which is soft. The mouth is surrounded by five groups of oral spines.

 2.    Ambulacral grooves:

From the comers of the mouth five ambulacral grooves will start and run along the middle of the arms.

 3.    Tube feet or Podia :

 Each ambulacral groove contains 4 rows of tube feet. They are soft and extensible tubular processes. Each tube feet end- in a sucker. The tube feet are useful for locomotion and food collection.

 4.    Ambulacral spines :

 Each ambulacral groove is guarded on each side by 2 or 3 rows of spines. These are movable. These spines are aggregated into five groups called Mouth papillae.

Three rows of stout immovable spines are present outside the ambulacral spines.

Another a row of spines will present along the borders of the arms separating the oral from the aboral surface.

 5.    Eye :

The eye is small and bright red spot. It is present at the end of each arm. It is light sensitive.

 6.    Tentacle. :

At the end of each arm a small non-retractile tentacle is present. It is olfactory in function.

 

Aboral surface or Abactinal surface :

 The surface of the star firsh which is facing upwards is called aboral surface. It is convex. It has the following structures.

 1.    Anus:

It is a small opening. It is nearly in the middle of aboral surface. It is slightly displaced towards the interradius.

 

2.    Madreporite :

It is a flat, almost circular plate. It is a sieve plate-like structure. It leads into a stone - canal of water vascular system. The madreporite is placed in an inter-radius of the central disc. It converts the radial symmetry of the animal into bilateral symmetry.

 Spines :

The spines of aboral surface are white in color. They are arranged in irregular radial rows. The spines are short, and stout. They are developed from the calcareous plates called ossicles. The ossicles are burried in the body wall and covered by the epidermis.

    Dermal branchiae orPapulae :

These are small finger-like processes. They come but through dermal pores. Dermal pores are present in between the ossicles. These papulae are also called gills. They are respiratory in function. They can also perform an excretory function. These papulae can extend or completely retracted into the body.

 Pedicellariae :

Pedicellariae are scattered all over the body. These are present in between the spines of the aboral surface. On the oral surface, they are present attached to the bases of the spines.

Each pedicellaria consists of a flexible stalk and three calcareous pieces—one basilar and two jaws or valves. The two jaws are attached to the basilar piece.

There are two types of pedicellariae 

1 Straight pedicellaria 

2. Crossed pedicellariae 

i) Straight Pedicellariae, where the two jaws remain more or less straight on the basilar piece.

(ii) Crossed Pedicellariae, where the basal portions of the jaws are curved and cross each other.

The pedicellariae are modified spines and are regarded as protective organs. The jaws are movable on the basilar piece, ope­rated by two sets of muscles — two pairs of adductor muscles to close them and one pair of abductor muscles to open them.