PALAEMON (PRAWN) DIGESTIVE SYSTEM
Palaemon's digestive system contains a long alimentary canal and large hepatopancreatic gland.
I. Alimentary canal:
- The alimentary canal is a long tube.
- It starts at the mouth and ends with anus.
- It shows buccal cavity, oesophagus, stomach intestine and rectum.
- The buccal cavity, oesophagus and stomach are lined with euncle.
- It is called stomodaeum or fore-gut. Intestine is lined by endoderm and is called mesenteron or mid-gut.
- The rectum is lined by cuticle and is called the proctodeam or hind gut.
- The mouth is a longitudinal slit on the ventral side of the head.
- It shows labrum on the anterior side.
- Mandibles are lateral.
- Thin labium is present on the posterior side.
b) Buccal Cavity:
- Mouth leads into the buccal cavity.
- It is short, and vertical. It shows thick and folded cuticle.
- The molar processes of the mandibles project into the buccal cavity.
- The buccal cavity opens into the oesophagus.
1) CIRCULAR PLATE
2) LANCEOLATE PLATE
4) CARDIAC STOMACH
5) GUIDING RIDGE
6) PYLORIC STOMACH
9) UPPER LIP
11) LOWER LIP
12) COMB PLATE
13) HASTATE PLATE
14) HEPATO PANCREAS
15) FILTERING APPARATUS
16) HEPATO PANCREATIC OPENING
- It is a short and wide tube.
- It's wall is folded. There are 4 longitudinal folds.
- Each lateral fold is sub-divided by a groove into two smaller olds.
- The cuticular lining of the oesophagus bears bristles.
- The oesophagus leads into the stomach.
- It is a large sac.
- It occupies more than half of the cephalothorax region.
- It is divided into large cardiac and small pyloric regions.
i) Cardiac Stomach:
- It is lined with a thin cuticle, h is longitudinally folded. At places the cuticle is thickened and calcified into plates.
- Near esophageal opening circular plate is present.
- On the roof near anterior end lanceolate plate is present.
- On the floor of cardiac stomach, hastate plate is present.
- Below the hastate plate a pair of comb plates is present. Lateral groove separates the hastate plate and comb plate.
- Each comb plate has a dense fringe of delicate bristles, which are directed inwards.
- Each lateral groove has a groove plate on the floor.
- Lateral longitudinal folds or guiding ridges are present one on either side of the comb plates.
- The cardio pyloric aperture is X-shaped and is bounded by anterior, posterior and lateral valves.
- The margins of the valves bear setae which act as a sieve. !t permits fluid or very fine food particles to pass into the pyloric stomach.
ii) Pyloric Stomach:
- It lies beneath the posterior part of the cardiac stomach.
- Its lateral walls are thick and project as large longitudinal folds into its lumen. They divide the pyloric stomach into a small dorsal chamber and a large ventral chamber.
- The lateral grooves on the sides of the hastate plate open into the ventral pyloric chamber.
- On the floor of the ventral chamber thick plates are present side by side.
- They are "V" shaped In cross-section. These two plates are called filter plates. On the ridge of the filter plate a row of bristles can be seen.
- The cuticle of the lateral walls of the ventral pyloric chamber and the filter plate forms a filter that permits only the fluid to pass through it.
- Behind the filter plate, pair of hepatopancreatic ducts will opens into ventral pyloric stomach.
e) Intestine: It is a long tube extends upto the 6th abdominal segment.
f) Rectum: It is very short. It extends from the 6th abdominal segment to the anus. Its anterior part is enlarged into a muscular sac'
g) Anus: It is present on the ventral side of the telson.
II. Hepatopancreas :
- It is a large orange-red gland.
- It consists to two separate lobes'and develops as a pair of outgrowths.
- The hepatic caecae. from the mid-gut.
- It lies around the stomach.
- It has many branching tubules held together by connective tissue.
- The tubules join to form larger tubes and they unite to form a pair of hepato pancreatic ducts.
- They open into the ventral pyloric chamber.
- It serves like the liver and pancreas of higher animals.
- It secrets digestive enzymes.
- It stores glycogen, fat and calcium like liver.
- It absorbs digested food from intestine.
Physiology: Palaemon is omnivorous. It eats algae, weeds, insect larvae, small fish and debris.
- Small food particles are caught by the chelate legs and pushed into mouth.
- The 3rd maxillipedes assist the chelate legs in handling large food masses.
- The coxae of the 2nd, maxillipede will hold the food.
- The incisor processes of the mandibles cut the food into small bits.
- Much of the digestion of food takes place in the cardiac stomach.
- It is brought by the digestive juice secreted by the hepatopancreas.
- Dissolved food pass into the lateral grooves and then into the ventral chamber of the pyloric stomach through the cardio-pyloric aperture.
- Digestion continues in the pyloric stomach.
- The food is filtered through the pyloric filter.
- Absorption of food takes place in the hepatopancreas and intestine.
- Major part of the digested food passes into the hepatopancreas through the hepatopancreatic ducts.
- The un-digested food particles left by the pyloric filter pass into the dorsal pyloric chamber.
- Then they go to the intestine.
Egestion : The undigested food is sent out through the anus.