Anatomy of Earthworm

Anatomy of Earthworm

1. Body Wall:

  • The external body wall of the earthworm consists of several layers, including a thin non-cellular cuticle, an epidermis, two muscle layers (circular and longitudinal), and an innermost coelomic epithelium.
  • The epidermis is a single layer of columnar epithelial cells that contain secretory gland cells.

 2. Alimentary Canal:

  • The alimentary canal is a straight tube that runs from the first to the last segment of the body.
  • It includes a terminal mouth, buccal cavity (1-3 segments), muscular pharynx, esophagus (5-7 segments), a muscular gizzard (8-9 segments), and a stomach (9-14 segments).
  • The earthworm's diet primarily consists of decaying leaves and organic matter mixed with soil.
  • The calciferous glands in the stomach neutralize humic acid found in humus.
  • The intestine starts from the 15th segment and continues to the last segment, with a pair of short and conical intestinal caecae projecting from the intestine on the 26th segment.
  • The intestine features an internal median fold of the dorsal wall known as the typhlosole, which increases the effective area of absorption.
  • The alimentary canal opens to the exterior through a small rounded aperture called the anus.

 

3. Circulatory System:

  • Earthworms have a closed circulatory system composed of blood vessels, capillaries, and a heart.
  • The closed system confines blood to the heart and blood vessels, with contractions maintaining blood circulation in one direction.
  • Smaller blood vessels supply the gut, nerve cord, and the body wall.
  • Blood glands located on the 4th, 5th, and 6th segments produce blood cells and dissolved hemoglobin present in the blood plasma. The blood cells are phagocytic.

 

4. Respiratory System:

  • Earthworms do not possess specialized breathing organs.
  • Respiratory exchange occurs through the moist body surface, allowing gases to diffuse into the bloodstream.

 5. Sensory System:

  • Earthworms lack eyes but have light and touch-sensitive organs (receptor cells) that detect light intensities and ground vibrations.
  • They also have specialized chemoreceptors (taste receptors) to react to chemical stimuli, primarily located on the anterior part of the worm.

 6. Reproductive System:

  • Earthworms are hermaphrodites, possessing both male and female reproductive organs.
  • They have two pairs of testes located in the 10th and 11th segments.
  • The vasa deferentia run up to the 18th segment and join the prostatic duct.
  • Accessory glands are present in the 17th and 19th segments.
  • A common prostrate and spermatic duct open to the exterior through a pair of male genital pores on the ventro-lateral side of the 18th segment.
  • Earthworms also have four pairs of spermathecae located in segments 6th-9th to store sperm during copulation.
  •  A pair of ovaries is attached to the intersegmental septum of the 12th and 13th segments, and ovarian funnels lead into oviducts.
  • These oviducts merge and open as a single median female genital pore on the 14th segment.
  • During mating, earthworms exchange packets of sperm called spermatophores and deposit mature sperm and egg cells, along with nutritive fluid, in cocoons produced by the gland cells of the clitellum.
  • Fertilization and development take place within these cocoons, which are deposited in the soil. The cocoon holds the worm embryos, and after about three weeks, it produces two to twenty baby worms.
  • Earthworm development is direct, with no larval stage.

 

 7. Excretory system

  • Excretory organs in earthworms are known as nephridia (singular: nephridium). These nephridia are classified into three types: septal nephridia, integumentary nephridia, and pharyngeal nephridia.
  • Septal nephridia are located on both sides of the intersegmental septa, starting from segment 15 and extending to the last segment. They open into the intestine.
  • Integumentary nephridia are attached to the lining of the body wall, starting from segment 3 and continuing to the last segment. They open directly onto the body surface.
  • Pharyngeal nephridia exist in three pairs, forming tufts in the 4th, 5th, and 6th segments.
  • Despite their different locations, all types of nephridia share a basic structural similarity.
  • Nephridia play a crucial role in regulating the volume and composition of the body fluids. 
  • They begin as funnels that collect excess fluid from the coelomic chamber. 
  • These funnels connect to tubular portions of the nephridia, which then transport waste materials through a pore to the body wall's surface and into the digestive tube.