Pre-oral chamber and mouth : The anterior or oral sucker on its ventral side possesses a cup-like depression, the pre-oral chamber, at the bottom of which is present a triradiate mouth. It is guarded by two dorso-laterals and one mid-ventral lip. 2.
Buccal Cavity: Behind the velum mouth opens into another short chamber, called buccal cavity. Three crescentic jaws, arranged in a triangle are embeded in the mucus lining of buccal cavity. Each jaw is a laterally compressed muscular structure covered with cuticle.
This cuticle is thickened at the edge and bears minute teeth or denticles in a single row. Such jaws are termed monostichodont. There are 103-128 denticles on the median jaw and 85-115 on the lateral jaws. On the flat sides of the jaw are nearly 42-45 button shaped salivary papillae, each with numerous openings of salivary glands.
All the three jaws act together to produce a triradiate bite or Y-shaped wound in the skin of host. 3. Pharynx: Buccal cavity leads into a highly muscular pharynx extending from 5th to 8th segment. Externally it is surrounded by large masses of unicellular pyriform salivary glands that open onto the jaw surfaces. They secrete hirudin or anticoagulin which prevents coagulation of blood while the leech sucks it from the host. Numerous radial muscles connect the wall of pharynx with the body wall. 4. Oesophagus : Pharynx leads into a very short and narrow tube, the oesophagus.
Its lumen is narrow and the inner lining is highly folded. 5. Crop: After oesophagus, the spacious thin walled portion of the digestive tract that extends from 9th to 18th segment is called crop. It is divided into 10 (sometimes 11) chambers, communicating with each other by more or less circular apertures guarded by sphinctors.
Each chamber extends laterally into storing pockets or caeca or divertiicula. They gradually increase in size, on moving downwards and the last 10th chamber is biggest bearing elongated coeca extended downwards along the intestine, upto the 22nd segment. Crop has the power of great dilation and is able to store enormous quantity of blood, which can be later digested slowly.
5. Stomach: The last chamber of the crop leads into a heart-shaped stomach through an aperture, guarded with a sphinctor. It lies in 19th segment and its mucus lining is thrown into a network of transverse folds. 7. Intestine: From 20th to 22nd segment a thin walled intestine is located in which stomach opens. Its inner wall has numerous spiral folds bearing microvilli to increase absorption surface. 8.
Rectum: Dilated tube, the rectum. Its inner lining is cuticular and smooth. It opens out mid-dorsally on the 26th segment through anus.