Alimentary Canal :
It is a simple & straight tube between the mouth & the anus. The earthworm exhibits tube within a tube arrangement because of the position of the alimentary canal.
The alimentary canal has following parts-
- Mouth : It is terminal & crescentric.
- Buccal cavity : It is found in first three body segments and is a protrusible structure which helps in locomotion & in the ingestion of food.
- Phrynx : It is a muscular & bulb-like structure which is found in 3rd & 4th segments. It’s wall is thick & muscular, and it’s dorsal side is comparatively more thicker. It’s dorsal side consists of unicellular salivary gland cells & special chromaffin cells. The salivary glands secrete saliva & the chromaffin cells are known to help in the synthesis of saliva. A horizontal shelf divides the lumen of the pharynx into two chambers viz.-
(a) Dorsal or Salivary chamber
(b) Ventral or Conduction chamber
The pharynx acts as a sucking organ due to presence of radial muscles.
- Oesophagus : It is a small & narrow’ tube which is found in 5th, 6th & 7th segments. Internally, it has many transverse folds.
- Gizzard : It is a bulb-like structure which is found in 8th segment. It’s wall has a thick layer of circular muscles. It’s cavity is lined by a columnar epithelium which secretes a hard cuticle lining. The cuticle consists of mainly chitin. The gizzard is meant to grind the food.
- Stomach : It is a thin & tubular structure which is found in 9th to 14th segments. It’s wall is glandular & it extends internally to form longitudinal folds. It has sphinctors at both the ends. The walls of the stomach has special calciferous gland cells. They secrete calcareous substance which neutralizes the humic acid & makes the medium alkaline.
- Intestine : It is a sacculated & tubular structure which extends irom 14th to last segment. It is divisible into three parts-
(a) Pre-typhlosolar region : It is found in 14th to 26th segments & it is without typhlosole. In 26th segment, it gives out a pair of intestinal caecae which extend upto the 23rd segment. The walls of the intestinal caecae are thin. According to Chen & Puh, the intestinal caecae secrete digestive enzymes.
(b) Typhlosolar region : This region is found from 27th segment to the segment except last 25 segments. It has a typhlosole which is an internal longitudinal fold in the mid-dorsal position. The typhlosole provides increased surface for the absorption of the digested food.
(c) Post-typhlosolar region: This part is found in the last 25 segments. It is without typhlosole. This part is also called as rectum.
- Anus : It is a longitudinal slit-like aperture.
