Sense organs in Asterias

Sense organs in Asterias

The senses are poorly developed and the sense organs in Asterias comprises of the neurosensory cells, terminal tentacles and the eyes.

Neurosensory Cells:

These are present scattered throughout the body surface, but are quite abundant in the suckers of the tube feet, in the ambulacral regions, in the epider­mis, at the base of pedicellariae and spines. Each neurosensory cell has a slender fusiform body, the proximal end of which is connected with sub-epithelial nerve plexus and the distal end goes up to the cuticle. These neurosensory cells are tactile in func­tion and act as chemoreceptors.

Terminal Tentacles:

These tentacles are regarded as tactile receptors. They help to survey the environment during locomo­tion.

Eyes:

The major sensory organs are the simple, pigmented eye spots situated one at the tip of each arm, on the oral side of the so- called terminal tentacle. The radial nerve cord ends at the base of the terminal tentacles as a sensory pad or cushion. This cushion is termed as the eye because it bears numerous photoreceptors in the form of cup-like pig­mented ocelli or eye pits

Externally, each ocellus, is covered by the cuticle, beneath which is present a lens-like transparent thickening formed by the epider­mis. This may be absent in some cases. The wall of the cup is composed of red pigmen­ted cells and retinal cells.

Each retinal cell ends in a small clear and highly refractile bulb, projecting into the cavity of the pit. A trans­parent gelatinous tissue fills the cavity of the eye pit. The number of ocelli per eye may be about 80-200 and the number may even increase with age.

Haemal System of Asterias or Sea star

 

Haemal System of Sea star 

In Asterias or sea star, the blood lacunar system as such is absent and its function has been taken up by a special system called haemal system, which consists of channels enclosed by coelomic spaces.

 

Axial Complex of Asterias:

A very well-formed axial complex com­prising of a coelomic cavity enclosing the stone canal and axial gland is present in Asterias.

The axial complex remains intimately associated with the inter-brachial septum. The axial sinus is a thin-walled tubular cavity which opens orally into the inner smaller ring of the hypo-neural ring sinus.

 

  

 

It opens aborally into the genital sinus and finally into the ampulla of the stone canal.

The axial gland is a long spongy body of brown or purple colour and gives a small terminal process at the aboral end. This process is surrounded by a contractile closed sac named as terminal sac or dorsal sac or madreporic vesicle. This vesicle is situated very close to the ampulla of the stone canal, but has no connection with it.

The axial gland terminates orally in the septum of the hypo-neural ring sinus. The axial gland is variously called as heart, ovoid gland, brown gland, septal organ and dorsal organ. The function of the axial gland and its relation to other systems is still disputed.

It has an interior core of connective tissue traversed by many spaces containing coelomocytes and externally it is covered by a coelomic epithe­lium.

Water Vascular system of Sea star

 WATER VASCULAR SYSTEM IN STARFISH

In Echinoderms a peculiar system is met with. It is filled with sea water, it is called water vascular system or Ambulacral system. The entire system is lined with ciliated epithelium. This system is well developed in Asteroidea.

It con­tains

1.      

1.     Madreporite

2.     Stone canal

3.     Ring canal

4.     Radial canals

5.     Tiedemann's bodies

6.     Lat­eral canals

7.     Tube feet.

 

Madreporite:

A calcareous plate-like structure is present on the aboral surface of the central disc of star fish. It is placed near to two arms and these two arms are called 'Bivium.' On surface of the madreporite grooves of furrows are present. Each furrow contains a number of pores. Hence it is called serve plate. These pores led into small canal which open into a stone canal.

Stone Canal:

It is an 'S' shaped canal, it is called the Madreporic canal. It travels towards the oral side on unite with ring canal at one of inter radius. The walls of the stone canal are strengthened by calcareous rings. The inner surface of the stone canal is lined with flagellated cells. The movement of flagella will draw water currents into the stone canal.

 

Ring Canal:

It is penta - radial and is present around the mouth, on the oral side. At each inner radius, it shows Polian vesicles and Tiedemann's bodies.

Pollan vesicle :

In the Asteroidea groups, four muscular like poiian vesicles are present at the 4 inter radi. Some scientists believe that they store water and are considered as reservoirs. Some scientists consider that they produce amoebocytes.

Tiedmann's bodies :

Usually in Asterias 9 Tiedeman's bodies are present. On either side of the polian vesicle two Tiedemann bodies are present. At the stone canal union with ring canal only one Tiedmann body is present. They are lymphoid sac like structures. They are believed to produce amoeboytes.

Radial canals

From the ring canal five radial canals arise and run throughout the entire length of the arm. Each radial canal lies below the ambulacra! the groove of the oral surface of the arm. Each radial canal ends at the tip of the arm as a tentacle. It is olfactory in function.

Lateral canals

 From the radial canal of each arm pairs of lateral canals will arise and they end with tube feet.

Tube feet:

Structure of Tube foot 

The basal part of the tube feet is bulged and is called the ampulla. It continues as a long tube feet which end with a 'Sucker'. The ampulla contains circular and longitudinal muscles. The long tube feet like structure contain only longitudinal muscles. Lateral canal will open into ampulla and is guarded by a valve.

Functions: Water vascular System: his mainly used for locomotion and food collection.

1)    Locomotors function:

The contraction of ampulla extends the tube feet. The tube feet will emerge out of the arm and attaches to the substratum with the help of suckers. The water from the ampulla will never enter the lateral canal because of the presence of valves.

The longitudinal muscles of the tube feet will contract and the animal is pulled forward and the Podia becomes short. The fluid is drawn back into the ampulla. Thus relaxation and contraction of tube feet will bring locomotory movement in Starfish. Usually, the locomotion is carried on by tube feet of one or two arms in one direction only.

2)    Food collection :

 In order to open the shell valves of the adhesion, Starfish utilizes its tube feet.-Then it feeds the soft material of Molluscan animals. Along each arm of the starfish extends an open ambulacral groove, from which arises a series of tube feet or podia. These are arranged in two rows and usually have suckers at their tip. Each tube foot has a coelomic cavity that communicates by a narrow neck with an ampulla lying in the main perivisceral coelom.

Actually the ampulla is a small muscular sac that bulges into the aboral side and opens directly in a canal that passes downward between the ambulacral ossicles and leads into the tube foot or podium. Each ampulla is associated with a tube foot and forms with it a func­tional unit.

Like the body wall, the podium is covered on the outside with a ciliated epithe­lium and internally with peritoneum. Between these two layers, lie connective tissues and longitudinal muscle fibers.

Role of Tube Feet in Locomotion: 

Mechanism of locomotion in Sea star (Tube Foot) 

In general, sea stars or sea star move rather slowly and tend to remain within a restricted area. In Asterias, the entire water vascular system operates as a hydraulic system dur­ing locomotion. In Asterias,  the circu­lating fluid enters each tube foot on the side of the ambulacral margin, and leaves it on the side of the ambulacral axis, while the current enters the ampulla on its aboral side and leaves it orally.

Each tube foot-ampulla unit may be cut off from the rest of the water vas­cular system by a valve, so arranged as to maintain the pressure developed within the unit. The functioning of the unit depends upon differences between the musculature of the ampulla and the tube foot .

In the ampulla the muscles consist mainly of rings of smooth muscles which are set verti­cally and lie parallel to the long axis of the arm. Contraction of these muscles brings about protraction of the tube foot and drives the fluid out of the ampulla into the foot.

The increase in the pressure is wholly translated into the elongation of the foot that subsequently comes in contact with the substratum. The musculature of the tube foot, in contrast to that of the ampulla, consists of longitudinal muscles, which are bounded on the inner side of the coelom. Contraction of these muscles is followed by withdrawal of foot from the substratum, bending and short­ening of the tube foot, and forceful return of the fluid back into the ampulla. All these make possible the highly organized stepping movement by which the sea star  pull themselves. 

Digestive system of Sea star

 

Digestive System And The Process Of Digestion In Star Fish

 

Starfish shows an alimentary canal extending from oral to aboral side. It shows the following parts.

 1. Mouth : 

It is present on the oral surface. It shows sphincter muscles. It, is capable of great expansion and redution.

2. Oesophagus: 

Mouth leads into short oesophagus. This leads into stom­ach.

3. Stomach: 

It shows cardiac and pyloric parts. The first part of the cardiac stomach is big and 5 lobed. It can come out through mouth. This is brought forward by 5 pairs of retractor muscles.

The pyloric stomach is small and star shaped. Each angle of the stomach is drawn into the corresponding arm. This is branched. These branches are called pyloric caecae or hepatic Caecae, or gastric glands. They work like pancreas. They secrete digestive juices. They store reserve food.

 

4. Intestine : 

Pyloric stomach opens into 5 sided broad intestine. From intestine 2 to 3 rectal caecae are given off.

The last part of the intestine is called rectum which opens out through anus.

 

Food: 

Star fish is a carnivorous animal. It feeds on a number of sedentary marine animals. Ex : Oysters, Crustaceans etc.

 

Feeding Mechanism :

Whenever it comes in contact with an oyster, it archs over it. With the help of tube feet, it separates the two valves of the oyster. Through this gap it pushes its cardiac stomach into the mantle cavity of the oyster.

 

Digestion :

Digestion is both extra and intracellular. The digestive juices are poured on the soft parts of the prey. The digestion is partly completed outside the body. Then starfish withdraws its stomach along with the digested food. The remaining digestion is completed" in the stomach and. pyloric caecae. The digestion is completed in the stomach and pyloric caecae. 

The digested food is absorbed through the walls of the digestive tract into the coelome and supplied to all parts. Most of the undigested food is sent out through mouth. In star fish very small amount of undigested food will be sent out through anus, hence the intestine and rectum are very small.

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.