In this article we will discuss about the dissection of prawn. Also learn about: 1. General Anatomy of Prawn 2. Dissection of Nervous System of Prawn.
General Anatomy of Prawn:
Take a well preserved prawn and after washing it thoroughly with running water hold it firmly in left hand. Take a very sharp scalpel (preferably one in which blade is affixed) and make a dorsal incision along the median line.
With the help of scalpel deepen this incision up to ventral surface so as to separate the animal into two halves. Of the two halves one would be having complete alimentary canal and nervous system and other might not be having these parts. Study the half which has nervous system and alimentary canal.
You will notice following parts:
1. Blood Vascular System:
On top of the half just below the extensor muscles you would observe the blood vascular system with all its components. All along the length of abdomen in the median line runs the supraintestinal artery.
This artery, as soon as it enters the cephalothorax is connected to the heart enclosed in a pericardial membrane. In-front, a pair of antennary artery and a single median ophthalmic arteries come out of the heart and are united with each other. Along the mid ventral line just below the nerve cord runs a ventral abdominal artery.
2. Alimentary Canal:
Below the supra intestinal artery runs a hollow tubular mid gut (intestine) which dialates in the region of telson into hind gut. Anteriorly, the intestine, passing through an orange coloured mass the hepatopancreas, dialates into a pyloric stomach, which is connected to a large bag like cardiac stomach.
The cardiac and pyloric stomach are also completely embedded in hepatopancreas. The cardiac stomach is connected to a small tube – the oesophagus, which opens into mouth surrounded by Jaws.
3. Nervous System:
The nervous system is represented by a median ventral nerve cord running in the median line just above the ventral abdominal artery at the base of flexor muscles. This nerve cord runs through the abdomen and in each segment has a ganglion.
As soon as it enters the region of cephalothorax it joins a large thoracic ganglionic mass. Anteriorly from this mass comes out a pair of circum oesophageal nerves, which unite into brain in between the two eyes. The two nerves are connected with a transverse commissure just below the cardiac stomach.
4. Reproductive Organs:
A pair of ovaries in female prawn and a pair of testes in male prawn are situated at the base of heart just inside the hepatopancreas. In addition there are present muscles – flexors and extensors and a renal sac (nephroperitoneal sac) above the stomach.
Dissection of Nervous System of Prawn:
Hold the prawn in left hand or put it in dissecting tray with its dorsal side upwards Make a lateral-cut, just above the hinge joints, on either side in all the abdominal segments as well as in carapace. Remove the tergal plates and median piece of carapace.
Remove the alimentary canal and blood vessel from mid dorsal groove. Now make a deep cut in the median line with the help of scalpel. Stretch the two sides laterally and pin them on sides. Locate the thick double, ganglionated and white nerve cord in the median ventral line just above sternal plates.
Now, start removing muscles from above the nerve cord and proceed in front along the nerve cord. Clear the thoracic mass along posterior edge of cephalothorax. Now, locate the two circumoephageal nerves coming out of the thoracic mass and running anteriorly up to the middle of the eyes, where they unite with brain. Gradually, remove the muscles from above the nerves with confidence and potions.
As soon as you reach the mouth or oesophagus, you will locate a hard chitinous saddle shaped structure. Just below it lies a transverse commissure and a small visceral ganglion. To expose them make a cut on either lateral side of the saddle shape structure and with the help of a blunt forceps slowly pull it up. You will see the transverse commissure just below it.
Now expose the brain or cerebral ganglion in between the eyes by removing a thin chitinous membrane from above the optic nerves and the brain. Now trace lateral nerves into walking legs and pleopods and remove all the muscles from around the nervous system. Black paper the dissection by placing small triangular black flaps below the nerve cord at short distances.
