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.
- 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, it 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. It 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.
- 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.
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.
The undigested food is sent out through the anus.