Anatomy of the Alimentary Canal
- The earthworm digestive system(Pheretima) is a complete, straight tube running the entire length of body. Mouth is located on the anterior end of the peristomium. It is over hung by prostomium and anus is located on the posterior end.
- Alimentary canal is divided into buccal cavity, pharynx, oesophagus, gizzard, stomach and intestine.
Feeding Mechanism in Earthworm(Detritivore Nutrition)
- The earthworm acts biologically as a detritivore, feeding on soil-rich organic matter.
- Its diet mainly includes decaying leaves, seeds, and microscopic algae present in the soil.
- Food intake occurs through a strong muscular pharyngeal pumping mechanism.
- The pharynx generates suction by coordinated contraction of specialized muscle fibers attached to the body wall.
- This mechanism helps in efficient ingestion of soil and organic debris mixture for digestion.
Mouth (buccal cavity)
The mouth leads into the buccal chamber. It extends upto the middle of 3rd segment. The crescentic opening located ventrally to the prostomium.
Buccal Chamber
The buccal chamber opens into the pharynx. It extends upto 3-4th segment and it is pear shaped. It is short, narrow, and protrusible. Its internal epithelial lining features prominent longitudinal folds.
Pharynx
- The pharynx leads into oesophagus. It extends upto the 4th segment. It is tubular. A pear-shaped, heavily muscularized chamber.
- Its anterior boundary is framed by a specialized nerve ring placed in transverse groove between it and buccal cavity.
- The cavity is dorsoventrally compressed due to presence of pharyngeal mass on the roof which produce salivary secretion.
- The lateral walls of pharynx are pushed inside forming a narrow horizontal shelf in each side.
- It consists of two chambers,namely a dorsal chamber (salivary chamber) and ventral is (conducting chamber). The two chambers are separated by partition called pharyngeal shelf. The salivary chamber contains a pharyngeal mass producing a salivary secretion.
- Salivary secretion contains mucus and proteolytic enzymes which are poured into salivary chamber.
Oesophagus
- The oesophagus leads into the gizzard.It is muscular oval sac. A short, thin-walled, narrow passage tube.
- It is occupied by the 7th segment and it is lined internally by the cuticle.
Gizzard
- Oesophagus is modified into hard, prominent, oval ,thick wall muscular organ known as Gizzard.
- It lies at 8th segment
Stomach
- It lies at 9-14 setments. It's wall is highly muscular and glandular and thrown into internal transverse folds.
- A narrow tubular organ containing specialized gastric glands.
Intestine
- The earthworm intestine is a long, wide ,thin absorptive tube divided into three distinct zones
- Based on the presence of the typhlosole (a dorsal absorptive fold) from the 15th segment to the last one.
- It's internal lining is ciliated , folded , vascular and glandular.
The Three Intestinal Regions
Intestinal Region Segment Range Distinctive Anatomical Features
Pre-Typhlosolar Region
- It lies at segments 15 to 26 Contains a pair of forward-directed, finger-like intestinal caeca at segment 26.
- From 26Segment are given out externally a pair of forwardly directed lateral conical outgrowth called intestinal caeca which run up to 22 or 23 Segment.
- These are highly vascular and villi like processes are present internally
- It's wall is highly vascular and internally folded to form specialized villi
Typhlosolar
- It is middle part which is lying between segments 27 up to 23-25 segments in front of anus. Highly glandular and vascularized; dominated by the typhlosole to maximize surface area.
- Arising as a median in growth of dorsal aspect of the intestinal cavity and it is called typhlosolar.
Post-typhlosolar
- It is the last part also known as rectum which is of about 23-25 Segment 90/95 to Anus The terminal, non-folded zone acting as a temporary storage rectum.
- It is internally marked by presence of longitudinal folds and it opens to the outside through a terminal anus
Histology of Gut Wall
Peritoneum-
- It is the outermost layer consisting of tall and narrow cells.
- The intestine and sometimes on stomach these cells contain yellow refractile granules called Yellow or Chloragogen cells.
- It is used for Storage of reserve food, Deamination of protein, formation of urea from ammonia, excretion etc.
Muscles-
- Lying below peritoneum include outer layer of longitudinal and inner layer of circular muscle fibers.
- It is well developed in pharynx and oesophagus but ill developed in intestine.
- In gizzard circular muscles are much developed while longitudinal muscles are absent.
- Muscles are unstriped or involuntary.
Enteric Epithelium-
- It consists of Single row of Columnar cell.
- It is ciliated on the roof of Pharynx.
- It is glandular in the Stomach
- It is absorptive and Glandular in the Intestine.
- It is internally thrown into folds in oesophagus, stomach and intestine.
Cuticle-
It is present in buccal cavity as a thin lining and in Gizzard as thick lining.
The Ingestion Phase: The earthworm acts as a detritivore, swallowing dead plant vegetation, organic debris, and soil. The muscular pharynx creates a strong vacuum pump action using muscle fibers attached to the body wall to pull fragments into the buccal cavity.
Pharyngeal Processing: No actual digestion happens in the buccal chamber. As food enters the pharynx, the pharyngeal mass releases salivary secretions containing mucin (for lubrication) and protease (to start breaking down proteins).
Mechanical Milling: Food passes through the thin-walled oesophagus into the oval, hard gizzard. The gizzard acts as a heavy-duty grinding mill, using powerful circular muscles (longitudinal muscles are absent here) and a thick cuticle lining to pulverize tough soil and food masses.
Acid Neutralization: In the stomach, specialized calciferous glands secrete an alkaline, chalky substance that neutralizes humic acids found in the ingested soil.
Final Breakdown & Absorption: The intestine serves as the principal site for chemical digestion. Glandular cells in the intestinal wall pour out a strong enzyme cocktail including pepsin, trypsin, amylase, lipase, and cellulase to completely break down organic nutrients.
Nutritional Component Target Substrate Active Enzyme
Involved Intermediate / Final Products Formed
|
Target Substrate |
Active Enzyme Involved |
Intermediate / Final Products Formed |
|
Proteins
(Step 1): Complex Proteins |
Pepsin |
Proteases + Peptones |
|
Proteins
(Step 2): Proteases + Peptones |
Trypsin |
Amino Acids (Final digestible unit) |
|
Carbohydrates: Starch |
Amylase |
Maltose |
|
Lipids /
Fats: Fats |
Lipase |
Glycerol + Fatty Acids |
|
Complex
Carbohydrates: Cellulose |
Cellulase |
Cellulobiose |