Digestive System of Earth Worm!
Alimentary Canal of Earth Worm:
The alimentary canal runs as a straight tube throughout the length of the body from the mouth to the anus.
It consists of the following regions:
1. Mouth and Buccal Chamber:
The mouth is a crescentic aperture lying below the prostomium. The mouth leads into a short, thin-walled tube called buccal chamber or buccal cavity running up to the 3rd segment. The lining of the buccal cavity is folded and it is surrounded by muscular strands. The buccal cavity can be protruded through the mouth with the help of special muscles that run backwards from the buccal chamber to the body wall.
The buccal chamber is followed by the pharynx which extends up to the 4th segment. It is a pear-shaped broad and muscular structure separated from the buccal cavity by a groove.
The roof of the pharynx is very thick and projects into the pharyngeal cavity as pharyngeal bulb, its lateral walls internally form two horizontal folds or shelves, one on each side, which divide the lumen of the pharynx into two chambers—the dorsal salivary chamber and the ventral conducting chamber.
The roof of the pharynx is lined by ciliated epithelium, above which are many muscles with connective tissues and blood vessels and outside these are the pharyngeal or salivary glands. These are groups of small, whitish unicellular glands of chromophil cells.
These glands open into the salivary chamber of the pharynx through their fine ducts. They secrete mucin for lubrication of food and also a proteolytic enzyme for the digestion of proteins. The ventral conducting chamber of the pharynx serves as the passage for the ingested materials.
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 dialate the pharyngeal lumen. Thus, the pharynx acts as a pump during feeding.
The mouth is pressed against the bits of humus of mud, then the pharynx undergoes a series of contractions resulting into the sucking of the food into the buccal chamber, and then pumping the same backwards into the oesophagus.
Behind the pharynx is a short, narrow, thin-walled oesophagus running up to the 8th segment. The oesophageal wall is folded internally and devoid of any gland.
The oesophagus is followed by the gizzard. The gizzard is a prominent oval, hard, thick-walled and highly muscular organ lying in the 8th or 8th and 9th segments. It has a thick wall of circular muscles lined by columnar cells which is further lined by cuticle. The gizzard grinds the food into a fine state.
The gizzard is followed by a short, narrow, thin-walled tubular stomach extended up to 14th segment.
The anterior and posterior openings of stomach are sphinctered. Its thin wall is highly glandular and vascularized but less muscular. Its internal wall is folded transversely. The glandular cells situated in the epithelial lining of stomach secrete a proteolytic enzyme. Some calciferous glands are found in the epithelial lining which produce calcium and CO2.
The calcium probably neutralizes the contents of the alimentary canal. Calciferous glands are excretory and remove ions of calcium and carbonate from the blood when the level of these ions becomes excessive, they are excreted as calcite into the stomach from where it passes out with mud through the anus.
The stomach is extending from 15th segment up to the anus. It has a beaded appearance due to constrictions corresponding to septa but bulging in each segment.
The lining of intestine has ciliated and glandular cells. The intestinal lining is folded to form villi, one of these villi becomes larger and well developed than the others to form the typhlosole. The typhlosole, thus, hangs in the lumen of intestine and runs mid-dorsally from 26th segment up to the last except posterior 24, 25 segments.
The typhlosole divides the intestine into three regions:
(i) Pre-typhlosolar Region:
The first part of intestine lying between 15th to 26th segments constitutes the pre-typhlosolar region. In this region the intestinal lining is folded to form villi which are highly vascular, no typhlosole is found in this region. In the 26th segment, two short and conical outgrowths, one on either side, are given off from the intestine. These are called intestinal caeca.
They are extended anteriorly up to 22nd segment and have a special blood supply; internally the caeca have many longitudinal folds which appear as villi in transverse sections, the epithelial cells being highly vascularized and filled with secretory granules.
Chen and Puh (1941) believe that these caeca are digestive glands and secrete an amylolytic enzyme for the digestion of starch. Usually active digestion occurs in this region.
(ii) Typhlosolar Region:
This is the second part of intestine lying between 26th to the last segment except posterior 24, 25 segments.
This region is provided with an internal median fold of the dorsal wall of intestine called the typhlosole, which is in fact, a well developed intestinal villi. The typhlosole increases the absorptive surface of the intestine. The process of digestion is probably completed in this region, hence, it is the major site of absorption.
(iii) Post-typhlosolar Region:
The intestine, in the last 24, 25 segment, has no typhlosole. It is the third region of intestine and is called the rectum. It is thin-walled, vascularised without villi and glandular cells. It contains small pellets of mud which are passed out from the anus as faeces at the opening of burrows. These are actually the castings of Pheretima.
It is a small circular opening placed at terminal end in the centre of the last or anal segment of the body.
Histology of the Alimentary Canal:
The alimentary canal is made of four layers; an outer layer of visceral peritoneum which forms the outermost covering layer of alimentary canal. The most of the cells of this layer around the stomach and intestine are modified and called chloragogen cells or chloragocytes containing yellow granules of chloragosomes.
Next to this, two muscle layers—outer layer of longitudinal muscle fibres and inner layer of circular muscle fibres are found.
The arrangement of muscle layers in the gut wall is just reverse to the body wall. Both these muscle layers are well developed around pharynx and oesophagus but poorly developed around the stomach and intestine. The longitudinal muscle fibres are absent in gizzard.
All the muscles of the gut wall are involuntary and un-striped. The last or fourth layer is the epithelial lining of columnar cells which are ciliated in the roof of the pharynx, in the stomach and intestine it is mostly glandular and ciliated to form villi in the intestine. In the gizzard, it mostly secretes cuticle.
Food and Feeding Mechanism of Earth Worm:
The earthworms are omnivorous; they feed upon all sorts of organic humus, decaying matters, small protozoans, nematodes, insects and other microorganisms found in the soil. They commonly ingest soil in large quantities and that is why their gut is always full of soil. During feeding the buccal chamber is everted out and the food is drawn into the mouth by sucking action of the muscular pharynx as stated earlier.
Digestive Glands of Earth Worm:
The pharyngeal or salivary gland cells, glandular cells of stomach, intestine and the intestinal caeca are supposed to be the various digestive glands which secrete the digestive enzymes for the digestion of food.
Physiology of Digestion of Earth Worm:
Various types of enzymes are said to be secreted by the digestive glands of earthworms for the digestion of food due to their omnivorous mode of feeding habit.
The swallowed soil passes through the buccal chamber into the pharynx where it receives the salivary secretion from the salivary gland cells. The salivary secretion contains mucin and a proteolytic enzyme. The mucin lubricates the food and food passage, while the proteolytic enzyme hydrolyses the proteins into peptones and proteoses.
Then the food comes into the gizzard through oesophagus. The gizzard further grinds the food and soil due to the action of the contraction of its well developed circular muscles into a fine state so that the enzymes of stomach, intestine and intestinal caeca can act upon it.
Firstly, after reaching the food in fine state into the stomach, it is subjected to the action of more proteolytic enzymes secreted by the glandular cells of this region.
This results in the conversion of proteins into peptones, as mentioned above. Then the food reaches into the intestine where it comes across the enzymes secreted by the glandular cells of intestine and intestinal caeca.
Usually, these enzymes are—proteases which hydrolyse peptones into amino acids, lipase hydrolyses fats into fatty acids and glycerol, amylases acting upon the carbohydrates and converting them into monosaccharide’s, cellulase hydrolyses the cellulose if any and chitinase hydrolysing the chitin present in the food. Digestion occurs in stomach and fully completed in the intestine.
The digested food is absorbed by the intestinal viili, more particularly by the typhlosole which increases the absorptive area of intestine. The absorbed food passes into the blood stream through the well developed capillary network of the intestine, from where transported to the tissues. The coelomic fluid also serves to transport the digested food to the tissues.
After digestion and absorption of food, the undigested remains and the soil are passed out through the anus as castings at the opening of the burrows.
As referred to earlier, that some of the cells of visceral peritoneum around the stomach and intestine are modified as chloragogen cells having yellow granules—the chloragosomes in them. These cells are believed to be of vital importance in the metabolism and they play a role similar to that of the liver in vertebrates.
However, these cells take up waste from the blood and probably from coelomic fluid and store it as yellow granules of chloragosomes. When these cells become filled with such granules, they either pass out through dorsal pores or excretory organs or the excreta remains permanently in chloragogen cells throughout life.
These cells are the chief centre of synthesis and storage of glycogen and fat. They manufacture glycogen from fatty acids and store it as reserve food and pass it on to the coelomic fluid when needed. Thus, these cells are excretory as well as they serve for storing the reserve food. These cells are also related with the deamination of amino acids, and the formation of ammonia, synthesis of urea, etc.