Earthworm: Digestive system

The digestive system of Earthworm consists of alimentary canal which is a straight tubular structure of varying diameter. It is complete as it has both mouth and anus. It remains differentiated in to following parts: mouth, buccal chamber, pharynx, oesophagus, gizzard, stomach, intestine and anus.

·        Mouth:-  It is anterior opening of alimentary canal and is present on ventral region of first segment. It is crescentric in shape and remains over hanged by prostomium.

·        Buccal chamber:-  Mouth opens in to a space called as buccal chamber which extends up  to middle of 3^rd segment. It is protrusible and internally remains lined by thin cuticle.

·        Pharynx:-  Buccal chamber leads into another chamber called as pharynx which extends from middle of 3^rd segment up to 4^th segment. In between buccal chamber and pharynx depression is present and on dorsal surface of this depression nerve ring lies. The cavity of pharynx remains compressed dorso-ventrally due to the presence of glandular pharyngeal mass on its dorsal roof. This pharyngeal mass produces salivary secretion which contains mucus and proteolytic enzyme. From lateral wall of pharynx fingerlike projections called as horizontal shelf. Due to the presence of horizontal shelf the cavity of pharynx remains differentiated in to dorsal or salivary chamber and ventral or conducting chamber. The roof of pharynx is ciliated.

·        Oesophagus:-  Pharynx leads in to a tubular structure called as oesophagus  which extends from 5^th segmentup to 7^th segment. Internally, it contains small folds and its function is to carry food from pharynx up to gizzard.

·        Gizzard:-  In 8^th segment oesophagus dilates to form an oval , hard and highly muscular structure gizzard. Internally, it remains lined by thick cuticle. It acts as grinding machine and its function is to pulverize food.

·        Stomach:-  Gizzard opens in to a tubular structure called as stomach which extends from 9^th segment up to 14^th segment. Internally, it is folded and glandular. The calciferous gland is present which produces calcium ions. At both ends of stomach sphincters are present to regulate amount of food entering in to stomach and passing out from stomach.

·        Intestine:-  Stomach leads in to another wide tubular structure called as intestine which extends from 15^th segment up to last segment. Externally, it seems beaded due to constriction of septum. Internally, it is folded, glandular and absorptive. It is primary site for digestion and absorption. The intestinal gland is present which produces intestinal juice which contains enzymes pepsin , trypsin, amylase, lipase and cellulase. The folds are commonly called as villi. One of the mid-dorsal villi lying from 27^th segment up to 23 or 25 segments in front of anus is large sized and is called typhlosole. Due to the presence of typhlosole  intestine remains differentiated in to three regions:

*Pre-typhlosolar region:- From 15^th segment up to 26^th segment and is without typhlosole. From lateral side of 26^th segment forwardly directed finger like projections arise and extends up to 22^nd or 23^rd segment and are called as intestinal caecum. They contain amylase producing cells.

*Typhlosolar region:- From 27^th segment up to 23 or 25 segments in front of anus and contains typhlosole which increases surface are of secretion and absorption.

*Post-typhlosolar region:-  In last 23 or 25 segments and is also called as rectum.

·        Anus:- In last alimentary canal opens outside by an opening called anus through which undigested food and ingested soil are passed out.

Histology of alimentary canal

Histologically, alimentary canal consists of following layers:-

·        Peritoneum:- It is outer most layer and consists of tall, narrow cells. Some cells of peritoneum in intestine contain yellow granules and are called as chloragogen cells or yellow cells. Sometimes these cells are also present in stomach. These cells convert glucose in to glycogen and store it, responsible for deamination and also help in excretion. They are like liver of higher organisms.

·        Musculature:-  Inner to peritoneum layer of muscles is present which remains differentiated in to longitudinal muscles and circular muscles. The longitudinal muscles extend along length and forms outer layer while circular muscles extend around and forms inner layer. The layer of muscles is well developed in buccal chamber, pharynx, oesophagus and gizzard while poorly developed in intestine.

·        Enteric epithelium:-  Inner to muscles, enteric epithelium is present which consists of columnar epithelial cells and is innermost layer in all parts of alimentary canal except buccal chamber and gizzard. It is folded in oesophagus, folded and glandular in stomach while folded, glandular and absorptive in intestine. It remains ciliated on the roof of pharynx.

·        Cuticle:-  It is present as inner most later in buccal chamber and gizzard. It is present as thick layer in gizzard while as a thin layer in buccal chamber.

Food , feeding and process of digestion

Earthworms feed upon dead-decaying organic matter and along with food large amount of soil is also ingested. They also feed upon leaves of small grasses and algae.

                   During feeding Earthworm brings its mouth in contact of food and the food is directly sucked in to buccal chamber by pumping action of pharynx as the pharynx acts as pump due to contraction and relaxation of muscles on its wall.

No digestion takes place in buccal chamber. From buccal chamber food passes in to pharynx where food get mixed with salivary secretion with mucus and proteolytic enzyme. The mucus lubricates food while proteolytic enzyme acts up on protein and converts it in to peptones and proteases. From pharynx food passes in to gizzard through  oesophagus. In gizzard food is pulverized by its grinding action. From gizzard food passes into stomach where food get mixed with calcium ions produced by calciferous gland which neutralizes acidic food as food contains humic acid. From stomach food passes into intestine where food get mixed with intestinal juice produced by intestinal gland which contains enzymes pepsin, trypsin, amylase, lipase and cellulose. The pepsin acts up on protein and converts it in to peptones and proteases. The trypsin acts up on peptones and proteses and converts them in to aminoacids. The amylase acts up upon carbohydrate and converts it into  glucose. The lipase acts upon fat and converts it into fatty acids and glycerol. The cellulase acts upon cellulose and converts it into cellulobiose.

These simple forms of foods are absorbed by inner lining of intestine. The presence of typhlosole  in most parts of intestine makes the process of absorption faster.

The undigested food and ingested soil pass out through anus in the form of worm castings which is like small round balls of soil.