Prawn: The prawn belongs to the phylum Arthropoda and class Crustacea. ltis familiar as a nutritious and tasteful food. There are different species of marine and fresh water prawns in Bangladesh. Galda prawn in a fresh water species. There are similarities among all the species of prawn in their internal and external structures. Bagda is a kind of marine prawn.
Species: M. rosenbergii
The whole body of a prawn is covered by an external covering or external skeleton made of substance known as chitin. The external covering or shell is divided into many segments where bands or spots are present.
The body of prawn in elongated, bilaterally symmetrical and spindle shaped.
The body of prawn is divided into two main parts, e.g.
A. Cephalothorax B.
Cephalothorax: Cephalothorax is again divided into two portions, e.g.
Head: Head is the anterior part of Cephalothorax. In the head there are fivepairs of segmented appendages, which are a. Antenulle b. Antenna c. Mandibles d. Maxillula and e. Maxilla.
Thorax: Just behind the head thorax is situated. Head and thorax togetherform the Cephalothorax. The first three pairs of appendages are Maxilla like and known as Maxillipeds. The last five pairs of thorasic appendages are used for walking. These legs are tubular elongated and segmented. First and
second pairs of walking legs are provided with forceps. The Cephalothorax is covered by a hard covering known as Carapace. At the anterior end ofcarapace there is a pair of black, round stocked compound eyes. In betweenthe eyes there is a laterally compressed flat saw like rostrum.
Abdomen: The elongated portion of the body behind the Cephalothorax isabdomen. It is round dorsally and a bit compressed laterally. The abdomen consists of six segments. In every segment of the abdomen there is a pair of appendages. They are used for swimming and known as Pleopods. The first five pairs of swimming legs are similar. All the legs are segmented.
Uropod: The sixth or the last abdominal appendages are uropod or tail fin.These are very big oar-shaped. At the end of abdomen the pointed portion is known as Telson.
Prawn is an invertebrate animal. So it is dissected on the dorsal side. The digestive system of prawn is formed of two parts, namely: 1. Alimentary Canal 2. Gastric Gland
l. Alimentary Canal: Alimentary canal is divided into three regions, namely A.Fore gut B. Mid gut C. Hind Gut.
A. Anterior Alimentary Canal or Fore Gut: It is divided in four parts, e.g.
i. Mouth Opening: It is a big opening on the ventral side of the head situated at the junction of 3rd and 4th cephalic segments. Food enters throwgh this opening.
ii. Buccal Cavity: Behind the mouth this short cavity is situated where foodcrushed.
iii. Oesophagus: A short and wide oesophagus is present at the back of buccalcavity from where food passes to the stomach.
iv. Stomach: It is a big sac like organ behind the oesophagus. The anteriorportion of the stomach is cardiac stomach and the posterior part is pyloric stomach. Food is digested in the stomach.
B. Mid Alimentary Canal or Mid Gut: This is straight, long, narrow andtubular portion of the alimentary canal. It is extended along the mid dorsal line up to the 6th abdominal segment. Its function is to absorb digested food and undigested food passes to the rectum through mid gut.
C. Posterior Alimentary Canal or Hind Gut: Behind the mid gut a small tube extended up to anus. It has two portions, namely:
i. Rectum: Small sac-like, first portion of posterior alimentary canal rectum.
ii. Narrow tubular posterior portion: It is located behind the rectum. At itsposterior end anal opening is present. The undigested waste materials are stored here. The faecal waste is eliminated through the anus.
2. Digestive Glands: The name of digestive gland of prawn is hepatopancreas,which is orange coloured and occupies most of the regions of Cephalothorax. It is a large gland surrounding the stomach. This gland functions like the liver, pancreas and small intestine of higher animals. They secret enzymes take part in digestion.
Take the prawn on hand, the covering be cut with scissors along the dorsal line-from the posterior end up to the anterior middle end. Then digestive system is to be observed carefully.