Digestive System of Earthworms

The digestive system of earthworms consists of the alimentary canal and the digestive glands.

Earthworm Alimentary Canal

• Complete straight tube throughout the length of the body from mouth to anus.
• Mouth constitutes its anterior and anus its posterior openings receptively.
• Functionally regionated into various parts i.e. mouth and buccal chamber, pharynx, esophagus, gizzard stomach, and intestine.

1. Mouth and Buccal Chamber

• Crescentic aperture situated ventral to prostomium.
• The mouth leads into a short, narrow, thin-walled protrusible buccal chamber.
• chamber extends up to the middle of 3 segments.
• Buccal cavity folded and surrounded by muscular strands.

2. Pharynx

• Followed by a buccal chamber.
• Extends up to the 4^th segment.
• Pear-shaped broad and muscular separated from the buccal cavity by a groove.
• The pharynx roof is thick and projected into the pharyngeal bulb.
• Pharyngeal bulb lateral walls internally form narrow horizontal shelves.
• Two shelves meet anteriorly and posteriorly and divide the pharyngeal cavity into the dorsal salivary chamber and ventral conducting chamber.
• The roof of pharynx lined by ciliated epithelium.
• Many muscles with connective tissues and blood vessels present above epithelium.
• Outside these present salivary glands
• Glands are small, whitish unicellular glands of chromophil cells.
• Glands open through fine ducts.
• Glands secrets mucin for lubrication of food and
• Proteolytic enzymes for digestion of proteins.
• The ventral conducting system of pharynx serves as a passage for the ingested materials.
• Like the buccal chamber, the pharyngeal wall remains to connect with the body by a thick mass of muscular strands.
• Contraction and relaxation of muscular strands serve to compress or dilate the pharyngeal lumen.
• Acts as a pump during feeding.
• Series of contraction of pharynx resulting in the suckling food into the buccal chamber and pumping the same back into the esophagus.

3. Esophagus or gullet

• Lined behind the pharynx.
• Short, narrow, thin-walled.
• Running up to 8^th segments.
• wall folded internally and devoid of any glands.

4. Gizzard

• modification of the esophagus into the prominent, hard, and thick-walled muscular organ.
• Lying in 8^th of 8^th or 9^th segments.
• the wall consists of circular muscles lined by the columnar cells.
• Columnar cells further lined by the tough cuticles.
• Grinds foods into fine states.

5. Stomach

• It is a gizzard followed by a short, narrow, and thin-walled tube.
• Extends up to 14^th segments.
• Anterior and posterior opening sphincter.
• Walls highly vascular and glandular but less muscular.
• Internal wall folded transversely.
• The epithelial lining consists of glandular cells and some calciferous glands.
• Glandular Cells secretes a proteolytic enzyme.
• Calciferous glands secrete calcium and CO2.
• Calcium neutralizes the contents of the alimentary canal.
• Calciferous glands are excretory removes ions of calcium and carbonates from the blood.
• Calcite excreted into the stomach when the level of ion becomes excessive and passed out with mud through the anus.

6. Intestine

• The region next to the stomach.
• Long, wide, and thin-walled tube.
• Extends from 15^th segments to the anus.
• Beaded appearance due to circular constriction corresponding to septa.
• The internal lining has ciliated and glandular cells.
• Internal lining folded to form villi.
• one of those villi become larger and well developed to form typhlosole.
• Typhlosole runs mis-dorsally from 26^th to last segments except 24, 25 segments.
• Divisible into 3 parts

1. Pre-typhlosolar region

• First or anterior parts lying between 15^th to 26^th segments.
• Walls folded internally to forms minutes process, the villi.
• Villi are highly vascular.
• No typhlosole is found in this region.
• 26^th segment gives outs externally a pair of forwardly- directed conical outgrowth, intestinal caeca.
• Intestinal caeca extended up to 22^nd or 23^rd segment
• Internally have any folds to form a villi-like process.
• Highly vascular and filled with secretory cells.
• Are digestive glands and secrete amylolytic enzymes for digestion of starch (Chen and Push (1941).
• In this region active digestion occurs.

2. Typhlosolar region

• Second or middle part of the intestine.
• Lies between 26^th to last segments except for 24, 25^th segments.
• Characterized by the presence of highly glandular and vascular longitudinal ridge.
• Provided with internal median folds of the dorsal wall of intestine i.e. Typhlosole.
• It increases the absorptive surface of the intestine.
• Process of digestion complete in this region.

3. Post- typhlosolar region

• Third or last parts i.e. 24, 25 segments.
• Has no typhlosole.
• Called rectum.
• Thin-walled, vascularized without villi and glandular cells.
• Casting occurs here.

7. Anus

• The small circular opening at the terminal end.
• Last or anal segments of the body.

Histology of Alimentary Canal

Peritoneum

1. Outermost layer.
2. Consists of tall and narrow cells.
3. Cells modified around the stomach and intestine called chloragogen cells or chloragocytes.
4. Cells contain yellow refractile granules called chloragosomes.
5. The exact function is controversial.
6.  But said to serve for the storage of food, deamination of proteins, deamination of proteins, the formation of urea from ammonia, excretion, etc.

Muscles

1. Lying below peritoneum.
2. Include the outer layer of the longitudinal and inner layer of circular muscle fibers.
3. They are well developed around pharynx and esophagus.
4. But poorly developed in intestines.
5. Muscles arrangements in the gut wall are just reverse to the body wall.
6. In gizzard, longitudinal muscles are absent.
7. But circular muscles are much developed.
8. All Gut wall muscles are involuntary and unstriped.

Enteric epithelium

1. Consists single layer of columnar cells
2. They become ciliated in the roof of the pharynx.
3. Mostly Glandular and absorptive in the stomach.
4. Glandular in intestine.
5. Internally thrown into folds in the esophagus, stomach, and intestine.

Cuticle

1. Present in the buccal cavity as thin lining
2. Present in the gizzard as thick lining.

Food and Feeding Mechanism

• The earthworm is omnivorous.
• Feeds directly upon organic humus, decaying matters.
• Also feeds directly upon leaves, grasses, seeds, small protozoans, nematodes, insects, algae, and other microorganisms found in soil.
• Ingest soil in larger quantities so, the gut is always full of soil.
• Ingest food by pumping action of its pharynx.
• The contractile sucking action of pharyngeal walls draws fragments of soil into the buccal chamber.
• Action is accelerated by the action of strands of muscle fibers, extending from pharynx to body wall.

Earthworm Digestive System Video

Physiology of Digestion

• Various types of enzymes are said to be secreted by digestive glands due to the omnivorous mode of feeding habit.
• Ingested food is pressed to move posteriorly.
• No digestion occurs in the buccal chamber.
• In the ventral conducting chamber of the pharynx, it receives the salivary secretion from salivary gland cells.
• Salivary secretion contains mucin and proteolytic enzymes.
• Mucin lubricates food and food passages.
• Proteolytic enzymes hydrolyze proteins into peptones and proteases.
• Then, food comes into gizzard through the esophagus.
• Gizzard acts as a grinding machine that further grind food and soil.
• This is facilitated by contractile movements of its muscular wall which causes the food to roll about, internal cuticle lining, striking against food particles are ground up fully.
• Then, Food reaches in the stomach in fine states.
•  chalky secretion of calciferous glands located in the stomach wall neutralizes the humic acid present in the soil.
• Then food reaches the intestine.
• The intestine is the principal site of digestion.
• Enzymes are secreted by glandular cells of the intestine and intestinal caeca.
• Enzymes like pepsin, trypsin, amylase, lipase cellulase and chitinase are secreted.
• Pepsin hydrolyzes proteins into proteases and peptones
• Trypsin hydrolyzes the product into amino acids.
• Amylases acting upon carbohydrates and converting them into monosaccharides.
• Lipase brings hydrolysis of fats into glycerol and fatty acids.
• Cellulase hydrolyses the cellulose into cellobiose.
• Chitinase hydrolyzes chitin present in food.
• Digestion is extracellular in the earthworm, as in higher animals such as frog and rabbits.
• Digestion occurs in the stomach and fully completed in the stomach.
• Intestine function for absorbing the digestive nutrients.
• Digested food is absorbed by intestinal villi, more particularly by typhlosole.
• Absorbed food are passed to blood capillaries in the intestinal wall.
• Coelomic fluid also serves to transport digested food to tissues.
• Undigested food and soils are passed out through anus as earthworm casting at the opening of burrows.
• The casting of earthworm consists of small and round pellets of balls.

References

1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.