The digestive system of leech consists of a straight tube called alimentary canal and associated salivary glands.
The alimentary canal of leech starts with mouth and ends with anus. It is divisible into the following parts.
- Fore gut or stomodaeum consisting of pre-oral chamber, buccal cavity and pharynx. They are lined with cuticle.
- Mid gut consists of oesophagus, crop,stomach and intestine. All these have endodermal lining.
- Hind gut or proctodaeum consists of rectum lined with cuticle.
Stomodaeum of fore gut:
- Pre oral chamber: The alimentary canal begins with mouth. The mouth is a small triradiate aperture situated in the centre of anterior sucker. It is guarded by velum made by three lips. The hollow space of anterior sucker present in front of the mouth is termed as preoral chamber.
- Buccal cavity: The triradiate mouth opens into the buccal cavity. Three crescent jaws are embedded in the mucous membrane of buccal cavity. One jaw is mid-dorsal in position and the other two are ventro-lateral in position. Each jaw is laterally compressed muscular structure. It is covered with a fine cuticle. It is thickened at the free edges to form a ridge. This ridge shows a row of minute teeth or denticles. Hence the jaw is termed as monostichodont. The median jaw has 103 to 128 teeth, while the lateral jaws bear 85 to 115 teeth on each jaw. On the either side of the jaw 40-45 salivary papillae with many openings of salivary glands are present.
- Pharynx: The next chamber is pharynx present in 5th to 7th segment. It is surrounded by salivary glands. The ducts of salivary glands open through the salivary papillae present on jaws.
Midgut or Mesenteron:
- Oesophagus: The pharynx opens into short narrow tube called oesophagus which opens into the crop.
- Crop: It extends from ninth to the eighteenth segments. It consists of ten segments. It consists of ten chambers, one in each segment. Each chamber is produced into 2 lateral diverticula. The diverticula of the last chamber are the biggest and reach up to twenty second segment. The crop acts as a blood storage organ of leech.
- Stomach: The crop enters into a comparatively small stomach present in the nineteenth- segment. The, opening of the crop into stomach is guarded by a sphincter.
- Intestine: The intestine is straight & narrow tube. It extends from the 20th segment to 22nd segment. Its mucous membrane shows longitudinal and transverse folds which increase the absorptive surface. The intestine leads into the rectum.
Hind gut or Proctodaeum:
- Rectum: It is broad and a short chamber lying in the 22nd segment to 26th segment.
- Anus: Rectum opens out through anus. It is very small opening present on the dorsal side of the body near the posterior sucker.
Leech shows 2 types of digestive glands. 1) Salivary glands, 2) Gland cells in the gut.
- Salivary glands: They are unicellular glands present below the pharynx. Each gland gives small duct. They open on the salivary papillae of the jaw. The saliva contains a substance called HIRUDIN. It prevents the clotting of the blood of the host during sucking.
- Gland cells in the gut: These are scattered in the epithelium of crop, stomach and intestine. They secrete mucous in the cop.
Food of Leech:
It sucks the blood of vertebrate animals. structurally leech is a sanguivorous animal.
Leech attaches to the host with the posterior sucker. Then it applies anterior sucker to the host body. The jaws move and make "A." shaped wound. The blood of the host is then sucked up by pump like actions of the pharynx. The salivary glands secrete a substance i.e. Hirudin, which prevents coagulation of blood. This enables the leech to draw blood continuously. A leech can suck blood which is 3 to 6 times more than its body weight. This is stored in the crop. When once leech drinks the blood, it need not go for sucking the blood till 3 months to one year.
The globin of the blood is the food of leech. The blood is digested in the stomach. The mechanism is not clear,
- In 1953, Bushing started the digestion is by bacteria in the gut.
- Heise (1909) started that a proteolytic enzyme is present in the gut.
The digested food is absorbed in the stomach and intestine.