In this article we will discuss about the dissection of star fish. Also learn about:- 1. Dissection of Alimentary System 2. Dissection of Ambulacral (Water Vascular) System.
The star fish has two surfaces. Aboral, the surface (Fig. 13.1) directed upwards in natural position and oral (Fig. 13.2) directed downwards. The two arms between which the madreporite lies are the bivium and the remaining three, trivium. Dissection of star fish is best done in specimens preserved in formalin or spirit.
Dissection:
Place the specimen on a dissecting tray with the aboral surface directed upwards. Put sufficient amount of water to cover it. Make a V-cut on the dorsal wall of the middle arm of the trivium near its tip with a sharp scalpel. Starting from the V-cut, cut the dorsal wall of the arm along its lateral borders up to the base with a pair taking care that the underlying structures of stout scissors. Cut the dorsal wall trans- are not damaged.
Starting from the cut at versely, close to the disc and remove it, the base of the arm, cut along the border of the disc. Give a second cut around the madreporite and a third cut around the anus. Carefully separate the disc from the aboral wall, taking care that the underlying structures, madreporite and the anus are not disturbed.
Dissection of Alimentary System:
With the removal of the dorsal wall of an arm and the disc, central portion of the alimentary system and the pyloric caeca of one arm are exposed. Pyloric caeca in other arms can be exposed following the same procedure. The alimentary canal is a short, straight, vertical tube, extending from oral to aboral surface (Fig. 13.3).
Mouth:
A five-rayed aperture, also known as actinostome, at the centre of the oral surface.
Oesophagus:
A short but wide tube connecting the mouth with the stomach.
Stomach:
It is divided into two by a horizontal constriction:
a. Cardiac stomach:
A voluminous five- lobed sac receiving the oesophagus.
b. Pyloric stomach:
Pentagonal in shape and dorsal to cardiac stomach. Each angle is drawn out to form a pair of pyloric caeca, which extend to the tip of the arm.
Intestine:
A short tube runs from the pyloric stomach to end in anus, close to the centre of the aboral surface.
Intestinal caeca:
Two hollow, branched caeca arise from the intestine.
Dissection of Ambulacral (Water Vascular) System:
The system (Fig. 13.4) is better dissected in a specimen injected with a non-diffusible dye. Remove the central disc and the aboral wall of one or more arms, without disturbing the madreporite, following the technique adopted in dissection of alimentary system.
Madreporite:
A flat disc with radiating grooves at the bases of the bivium.
Stone canal (Madreporic canal):
A S-shaped, vertical, cylindrical tube, supported by calcareous rings, runs downward from madreporite to join the ring canal.
Ring canal:
A pentagonal vessel around the mouth.
Tiedmann’s vesicles:
Nine, small outgrowths on the medial side of the ring canal. Radial canals (Ambulacral vessels). Five in number, arise from the ring canal and run distally, one in each arm along the roof of the ambulacral groove.
Tube feet:
Each radial canal gives out a series of lateral canals. Each lateral canal divides into two, to form the ampulla and the tube foot. The tube foot projects out through the ambulacral pore while the ampulla remains within the body.