The digestive system of an earthworm consists of the alimentary canal and the digestive glands.
The alimentary canal is complete in earthworm which is a long tube with variable diameter. Alimentary canal includes following parts:
Mouth and buccal chamber:
The mouth is situated in the first segment called peristomium. It is crescentic aperture lying on the ventral side of a first segment called peristomium and dorsal to it lie prostomium. The mouth leads into a short, thin-walled tube called the buccal chamber.
The buccal chamber is a thin-wall chamber which extends up to the 3rd segment. It can protrude out and retract in due to contraction of muscles which is attached to the body wall. It helps in ingestion of food.
The pharynx is pear-shaped, the thick-walled muscular organ which lies up to 4th segment and is separated from the buccal chamber by a dorsal groove where lies the brain of an earthworm. The roof or dorsal part of pharynx bear pharyngeal mass or pharyngeal bulb which is made up of muscle fibre, connective tissue, blood vessels and a group of a unicellular salivary gland called chromophil cell. The pharyngeal wall remains in connection with the body wall by a thick mass of muscular strands like the buccal chamber. The contraction and relaxation of these muscular strands serve to compress or dilate the pharyngeal lumen.Thus pharynx act as a pump during feeding.Pharynx:
The oesophagus is a very short thin-wall tube which extends from the 5-7 segment. It helps in the conduction of food materials.
Gizzard is an oval, hard, thick-walled muscularized organ which lies in the 8th segment and sometimes may extend up to the 8-9th segment. It is called grinding machine and helps in mechanical digestion. It has the inner lining of cuticle which is secreted by epithelium and outside to it the thick layer of circular muscle fibre is present.
The stomach is narrow, short, thin walled tube which extends from the 9-14th segment. Both the ends have sphincter. The inner lining is folded, vascular and glandular. The glandular part secrete proteolytic enzymes (proteases) and the calciferous gland secrete calcium and carbon dioxide. Calcium helps to neutralize the acidic food. Excess of carbon dioxide combined with calcium forming calcite are excreted out with mud through the anus.
The intestine is long, wide, thin-walled, tubular in the structure extending from 15 segments to the last segment anus. The inner lining is glandular, vascular, ciliated and folded to form intestinal villi. It is divided into three parts which are described below:
The pre-typhlosolar region lies from the 15-26 segment. In this region, the intestinal line is folded to form intestinal villi. From 22 segment to 26 segments lateral conical projection is seen which is known as intestinal caeca. Intestinal caeca secrete amylase and help in digestion.
The typhlosole region lies after the 26th segment to last segment except 25th segment. Its inner lining has intestinal villi and median mid-dorsal longitudinal fold of intestine is called typhlosole which is supplied with the blood vessel and increase the absorptive surface of the intestine.
Post-typhlosole region is last 25 segments of intestine. It is also called rectum. It does not have intestinal villi. It helps to absorb water and minerals.
It is the vertical slit present on the last segment through which undigested food materials are thrown out of the body.
Various types of enzymes are secreted by the digestive glands of earthworm for the digestion of food. Mouth and buccal cavity play the role in ingestion. The buccal cavity is exerted out and the food sucked in due to sucking action of the pharynx. The salivary glands of pharynx secrete saliva which contains mucin and proteolytic enzyme. The proteolytic enzyme acts on protein and breaks into peptones and proteases. Mucin helps in the lubrication of food. Oesophagus helps in the conduction of food materials towards gizzard.
In gizzard mechanical digestion takes place and the food is grinded into finer state with the help of sand grains and tough layer of cuticle. The contraction and expansion of gizzard are due to thick layer of circular muscle fibre. The stomach wall contains different types of the gland which secrete proteolytic enzyme and in the same way the calciferous gland secrete carbon dioxide and calcium which neutralizes the acidic food and may also form crystals of calcites. Protein is acted upon by pepsin and converted to peptones and proteoses.
Most of the digestion in earthworm take place in the stomach and fully completed in the intestine. An intestinal wall having glands secrete intestinal juice having proteases like pepsin, trypsin, amylase, maltase, lipase, chitinase, cellulose. Peptones and proteoses are further broken by trypsin and is converted into the amino acid. The digested food is absorbed by typhlosole and intestinal villi as they are richly supplied by blood vessels. The absorbed food is then utilized for generating energy by oxidation and the excess of the food substance is stored and converted into protoplasm which is utilized for growth and repair.