• Details on digestive and excretory system of earthworm

Structure of Alimentary canal of Earthworm

• Alimentary canal is a long straight tube extending from the first segment to the last segment of an earthworm’s body.
• It represents a tube within a tube body plan.
• It begins with an anterior mouth or prostomium and ends in the posterior anus.
• Along with it, it includes buccal cavity, pharynx, oesophagus, gizzard, stomach and intestine.
• Mouth:
  • It is a semicircular structure situated in the first segment called the peristomium just below the hood like prostomium.
  • It is highly elastic in nature and can protrude out and in.
  • Buccal/oral cavity:
  • It lies within the second and third segments.
• Pharynx:
  • Following the buccal cavity, the pharynx lies in the 4^th, 5^th, and 6^th segments.
  • It is thick and highly muscular.
  • The pharyngeal cavity is divided into two types:
  • Dorsal chamber (ciliated): They possess salivary glands in the outer region, that are formed by the chromophil cells (secrete saliva, i.e., mucus) and proteolytic enzymes for the digestion of the proteins.
  • Ventral chamber (non-ciliated): It is non-glandular and is termed as conducting chamber.
  • The effective organ for food digestion in earthworm is the pharynx.
• Oesophagus:
  • It extends from 5^th to 7^th segments.
  • They are quite thin, non-glandular and do not possess any muscular folds.
• Gizzard:
  • It lies in the 8^th and 9^th segments.
  • It acts as grinder hence termed as grinding machine, i.e. it aids in grinding of soil particles mixed with organic matter and other decayed materials.
  • The wall of the gizzard is made up of an outer layer of the circular muscles, a single layer of epithelial cells and an innermost thick layer of cuticle that is secreted by epithelial layer.
  • The contraction and relaxation of circular muscles cause the easy mastication of food and soil.
• Stomach:
  • The gizzard leads to the stomach, that is present from second half of 9^th segments upto 14^th segments.
  • It is a longer tube with short narrow cavity termed as glandular chamber.
  • The wall of stomach consists of calciferous glands whose secretion causes the neutralization of acidity of soil.
• Intestine:
  • It extends from 15^th to the last segment.
  • An internal long fold of dorsal wall is present after the 26^th segment which is termed as typhlosole.
  • Typhlosole is responsible for increasing the surface area of absorption.
  • Intestine can be divided into three types on the basis of typhlosole:
  • i) Pre-typhlosolar region:
  • It extends from 15^th to the 25^th segment.
  • It is highly glandular.
  • A pair of short and conical intestinal caecae is present on the 26^th segment.
  • ii) Typhlosolar region:
  • It lies from 26^th to 75-95last segment.
  • It is glandular and is highly absorptive.
  • iii) Post-typhlosolar region:
  •  It is the region after typhlosole.
  • It lies in the last 23-25 segments (76-96 to 100-120).
  • It is also termed as rectum and is absorptive in nature.
• Anus:
  • It is present in last/anal/pygidial segment.
  • It is a vertical, slit-like, small terminal aperture.
  • The defecation of worm-castings occurs through this aperture.

Physiology of digestion in Earthworm :

• In pharynx the food is mixed with saliva secreted by glandular cells of pharyngeal bulbs.
• Saliva contains mucin and proteolytic enzymes.
• Within gizzard, food is grinded into fine powder. The main reason for ingesting soil is to produce friction during breakdown of food.
• From gizzard the food reaches into stomach.
• The gland cells of stomach secrete proteolytic enzymes, which convert proteins and small peptides into amino acids.
• In stomach, the neutralization of food by calcites (CaCO₃) and digestion of rest of the proteins takes place.
• In intestine complete digestion of proteins, polysaccharides, fat, chitin, and cellulose takes place.
• Proteins when acted by proteases break down to peptones and proteoses.
• Further, peptones and proteoses are acted upon by proteases to form amino acids.
• The amylase enzyme is responsible for breakdown of polysaccharides into disaccharides.
• The lipase enzyme converts fats or lipids into fatty acids.
• Cellulose is also converted to disaccharides by lipase.
• Chitin is converted to disaccharides by lipase.
• After completion of digestion, both the digested and undigested foods pass to typhlosolar region.
• Here, the digested substances are absorbed by typhlosole and are circulated throughout the blood vascular system.
• The portion of food and soil that remained, passes to post typhlosolar region for storage.
• Finally, such substances are removed from the anus in the form of castings.

Excretory system of Earthworm:

• The nephridia are the excretory organs of earthworm.
• They are ectodermal in origin.
• Nephridia are analogous to kidneys of vertebrates.
• Nephridia are present in all segments of the body except in first 3 segments and last segments.
• In earthworm, the nephridia functions for the removal of the excretory wastes both from blood and the coelomic fluid.
• There are 3 types of nephridia based on the structure and location:
  • Septal nephridia or typical nephridia: Enteronephric nephridia
  • Integumentary nephridia: Exo-nephric nephridia
  • Pharyngeal nephridia: Enteronephric nephridia

1. Septal nephridia (Enteronephric nephridia):

• Septal nephridia are the largest nephridia.
• They are located in both sides of septum in each segment, behind the 15^th to 2^nd last segment.
• Each inter-segmental septum possess two rows of septal nephridia numbering 80-100 on each septum.
• Each septal nephridium has 4 parts: Nephrostome (nephridiostome or ciliated funnel), neck, body of nephridium and terminal duct.
• Septal nephridium is distinguished from pharyngeal nephridia in having nephrostome.
• Septal nephridia are the only nephridia with nephrostome or funnel.
• The terminal duct opens into septal excretory canal.
• These canals in turn open into two supra-intestinal excretory canals, so called enteronephric nephridia.

2. Integumentary nephridia:

• These are smallest nephridia.
• These are V-shaped in structure and are the most numerous types of nephridia.
• Integumentary nephridia are scattered in the body wall in all segments except in the first 7 segments and last segment.
• In each segment, there are about 200-250 integumentary nephridia.
• However, in the clitellar segments, the number is 10 times more than in ordinary segments.
• Hence, clitellar region is also termed as the forest of nephridia.
• As the terminal duct of integumentary nephridium is internally closed, each nephridium opens upon the body surface through nephridiophores.
• Hence, these nephridia are referred as exonephric nephridia.
• No nephridiopores are found in integumentary nephridia.

3. Pharyngeal nephridia:

• Pharyngeal nephridia (3 pairs) occurs as paired tufts on either side of pharynx and oesophagus.
• One pair each is present in 4^th, 5^th, and 6^th segments.
• Each of these tufts comprises of hundreds of pharyngeal nephridia as coiled tubes only.
• The terminal duct of nephridia of each tuft open into a common pharyngeal nephridial duct or the common excretory duct.
• Thus, there are three pairs of common pharyngeal nephridial duct.
• They run upward parallel with ventral nerve cord and open into alimentary canal.
• Ducts from 4^th and 5^th segments open into the pharynx in 4^th while those from 6^th segment open into buccal cavity in 2^nd.
• As these nephridia directly open into gut (buccal cavity and pharynx), they are of enteronephric type.
• Pharyngeal nephridia are also termed as pepto-nephridia.
• Earthworms are mainly ureotelic as their chief excretory product is urea (urea-50% and ammonia-45% and other 5%).
• The chloragogen cells excrete silicates consumed along with the food by Pheretima.

Summary points on Earthworm digestive and excretory system:

• The alimentary canal comprises of mouth, buccal cavity, pharynx, oesophagus, gizzard, stomach, intestine and anus.
• Pharynx lies in the 4^th, 5^th, and 6^th segments.
• The role of typhlosole is to increase the surface area of absorption.
• Gizzard acts as a grinder for soil particles along with food.
• Various enzymes like amylase, proteases and lipase are engaged during digestion.
• Pharynx is the effective organ for digestion.
• Nephridia are the excretory organs for earthworm and are of three types.
• Septal nephridia are the largest nephridia and integumentary nephridia are the smallest ones.
• One pair each of pharyngeal nephridia is present in 4^th, 5^th, and 6^th segments.
• Earthworms are mainly ureotelic.

Digestive and excretory system of Earthworm

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