Study of external characters and digestive system of Starfish.

Title:- Study of external characters and digestive system of Starfish.

Asterias is marine & lives on moist sea coast. It is bottom dwelling hence called benthoic animal. It is carnivorus & voracious feeder.

• Systematic position :-

Phylum - Echinodermata

Sub-phylum - Eleutherozoa

Class - Asteroidea

Order - Forcipulata

Family - Asteridae

Genus- Asterias

Species - rubens

• External characters :-

1)Shape & size - The body of starfish is star shaped, flattened in oral, aboral axis & radically symmetrical, Pentamerous arrangement. Consist , Pentagonal disc from which radiate out five, elongate, tapering arms. The axes of arms are known as radii & the regions of Central disc between the arms are termed as inter-radii. The body has an oral surface on which mouth is situated. This surface normally kept towards the substratum. The aboral surface is Convex & Covered with spines of various length. A minute opening is situated at centre called Anus.

The surface is also bears madreporite at inter radii position.

2)Colour:- Asteria is usually bright yellow, brown or orange coloured.

3)Oral Surface :- The surface normally kept towards the substratum is called oral or actinal surface. It bears mouth, ambulacral grooves, ambulacral Spines, tube feet, eyes & Tentacles.

i)Mouth - It is a circular aperture situated at the centre of oral surface of central disc. It is guarded by five groups of oral spines or mouth papilla.

ii) Ambulacral groove - From the five corners of mouth or Actinosome radiate out five narrow grooves called ambulacral grooves; which runs along the middle of each arm up to it's tip. Each groove shows two rows of tube feet.

iii)Ambulacral spines - Each ambulacral groove is bordered & guarded from the lateral sides by 2 or 3 rows of movable calcarious ambulacral spines. These spines are capable of closing over the groove. 

iv)Tube feet or podia - Each Ambulacral groove Contains two double row of soft, thin walled, extensible tubular structures called tube feet, Each tube feet has sucker disc; podium (middle) & ampulla (upper sac).

4)Aboral Surface :- The upper convex surface is called aboral or Abactinal surface. It bears spines dermal branchiae, anus, madreporite & Pedicellariae. 

i)Spines - The entire aboral surface is covered by short, stout, blunt & immovable calcarious spines or tubercles.

ii)Dermal branchiae - Those are very small, soft, delicate, hollow, finger like membranous retractile process present between the oscicles of integument called dermal branchiae or gill or papula. Papula is hollow evagination of body wall. The dermal branchiae are respiratory in function. 

iii)Anus- It is a small aperture, lies nearly in the centre of aboral surface.

iv)Madreporite- It is flat, round, small but conspicuous button like structure called madreporite. It is situated on aboral surface eccentrically. The two rays between which madreporite is present are called bivium & three remaining rays Trivivum. The madreporite is sieve, like Porous places &  leads to stone canal of water vascular system. 

v)Pedicellariae - Pedicellariae are modified spines that occur in the space between the spines all over the body. These are microscopic pincer-like or jaw like bodies. Each Pedicellariae consist basal Stalk which bears calcarious plates or oscicles & two joins or valves, pedicellariae are protective in function.

• Digestive System of Starfish :-

i)Mouth - A five- rayed aperture, Present at the centre of oral surface. It is also called Actinosome.

ii)Oesophagus - A short but wide tube connecting mouth with stomach.

iii)Stomach - It is divided into two by horizontal Constriction, the lower Cardiac & upper pyloric stomach. Stomach is largest part of alimentary canal.

a)Cardiac Stomach - It is spacious, five lobed sac, occupy the greatest part of central disc. The wall of this is thin, muscular & highly folded. It can be completely everted through mouth by Pressure of Coelomic fluid.

b)Pyloric Stomach - It is smaller, Pentagonal sac in Communication with Cardiac stomach dorsally. Each angle of Pyloric stomach is drawn out into a duct while enter in arm & branches to form a pair of large appendages, called as pyloric caeca or hepatic caeca or gastric glands. Thus, there are five pairs of pyloric caeca, one Pair in each arm.

iv)Intestine :- A short, narrow tube runs from the Pyloric stomach to end in Anus. It gives off 2 or 3, little hollow diverticula called Intestinal or rectal caeca before opening the anus. The rectal glands are brown in colour & probably excretory in function.

v)Anus - Intestine opens on the aboral surface by Small opening on the central disc Called Anus. It is slightly away from the centre.

vi)Digestive glands - Five pairs, long, brownish or greenish bodies, in each pyloric caecum, the hollow axis gives off laterally two series of small bollow branches, each terminating into a number of small bladder like Pouches or lobules.     Digestive glands are concerned with secretion of digestive juice containing proteases, amylases & lipase enzymes.

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