It is composed of
- Outermost thin non-cellular cuticle.
- Epidermis: Made up of a single layer of columnar epithelial cells which contain secretory gland cells.
- Two muscle layers (circular and longitudinal).
- An innermost coelomic epithelium.
- The straight alimentary canal extends from first to last segment of the body. It has
- Mouth → buccal cavity (1-3 segments) → muscular pharynx (4th segment) → oesophagus (5-7 segments) → muscular gizzard (8-9 segments) → stomach (9-14 segments) → Intestine (15th segment to last) → anus.
- Gizzard helps to grind soil particles, decaying leaves, etc.
- Calciferous glands, present in the stomach, neutralise the humic acid present in humus.
- A pair of short and conical intestinal caecae project from the intestine on the 26th segment.
- The intestinal part between 26-35 segments has an internal median fold of dorsal wall called typhlosole. It increases area of absorption.
- The organic rich soil is digested in the digestive tract by digestive enzymes. Digested nutrients are absorbed through intestinal membranes. Their faecal deposits are known as worm castings.
Circulatory system (blood vascular system)
- Circulatory system is closed type (blood flows through heart and blood vessels).
- Consists of blood vessels, capillaries and heart.
- Contractions keep blood circulating in one direction.
- Blood glands are present on the 4th, 5th & 6th segments. They produce phagocytic blood cells and haemoglobin which is dissolved in blood plasma.
- No specialized system.
- Gas exchange occurs through moist body surface into the blood stream.
Excretory organs are segmentally arranged tubules called nephridia. They are 3 types:
a. Septal nephridia: Found on both sides of intersegmental septa (segment 15 to last) that open into intestine.
b. Integumentary nephridia: Attached to lining of body wall (segment 3 to last). They open on body surface.
c. Pharyngeal nephridia: Present in the 4th, 5th & 6th segments in the form of paired tufts.
Funnel-shaped part of nephridium collects excess fluid from coelom. The funnel connects with a tubular part of nephridium which delivers the wastes into digestive tube.
- Includes segmentally arranged ganglia on the ventral paired and fused nerve cord.
- The nerve cord in the anterior region (3rd & 4th segments) divides and encircles the pharynx and joins the cerebral ganglia dorsally to form a nerve ring.
- The nerve ring with cerebral ganglia represents the brain.
- Sensory system: Includes
§ Light and touch sensitive receptor cells. No eyes.
§ Chemoreceptors (taste receptors): React to chemical stimuli.
Earthworm is hermaphrodite.
Male reproductive organs:
§ Testes: 2 pairs. Enclosed in testis sacs in 10th & 11th segments. The sperms from testes shed into testis sacs. From where, they enter seminal vesicles for maturation. Mature sperms move back into testis sacs and enter spermiducal funnels which are connected to vasa deferentia (spermatic ducts). The vasa deferentia run up to 18th segment where they join the prostatic duct.
The common prostate and spermatic duct open to the exterior by a pair of male genital pores on the ventro-lateral side of the 18th segment.
§ Accessory glands: 2 pairs. Found in the 17th & 19th segments (one pair in each segment).
Female reproductive organs:
§ Spermathecae: 4 pairs. Located in 6th-9th segments (one pair in each segment). They receive and store spermatozoa during copulation.
§ Paired ovaries: Attached at the inter-segmental septum of the 12th and 13th segments.
§ Ovarian funnels: Present beneath the ovaries which continue into oviduct, join together and open on ventral side as single median female genital pore on 14th segment.
- During mating, two worms exchange sperms each other. They mate juxtaposing opposite gonadal openings exchanging spermatophores (packets of sperms).
- Mature sperm, ova and nutritive fluid are deposited in cocoons produced by gland cells of clitellum. Cocoon with fertilized ova slips off and deposit in the soil. After 3 weeks, cocoon produces 2 to 20 baby worms (no larva).
- Earthworms are known as ‘friends of farmers’ because they make burrows in the soil and make it porous which helps in respiration and penetration of the plant roots. This process of increasing fertility of soil is called vermicomposting.
- They are used as bait in game fishing.