Anatomy of Earthworm
Body Wall: The body wall of earthworm is covered by thin non-cellular cuticle. Epidermis lies below the cuticle. This is followed by two layers of muscles. The outer layer is composed of circular muscles, while inner layer is composed of longitudinal muscles. The innermost layer is the coelomic epithelium. The epithelium is composed of a single layer of columnar epithelial cells. The epithelial cells contain gland cells as well.
Alimentary Canal: The alimentary canal is a straight tube. It runs between the first to last segment. Mouth is terminal and opens into the buccal cavity (1- 3 segments). The mouth leads into muscular pharynx.
Pharynx continues into oesophagus (5-7 segments) which is a small narrow tube. Oesophagus is followed by a muscular gizzard (8-9 segments). The gizzard helps in grinding the food. The stomach extends from 9th – 14th segments.
Decaying leaves and organic matter, mixed with soil, are the foods of the earthworm. The humic acid, present in humus, is neutralized by the calciferous glands in the stomach.
The intestine continues from the 15th segment to the last segment. On the 26th segment, a pair of short and conical caecae project from the intestine. Between 26th -35th segments, internal median fold of dorsal wall is present in the intestine. This internal fold is called typhlosole. The typhlosole increases the area of absorption in the intestine.
The alimentary canal opens to the exterior by a small rounded aperture; called anus.
Blood Vascular System
Closed type blood vascular system is present in earthworm. The blood vascular system is composed of a heart, blood vessels and capillaries. Smaller blood vessels supply the gut, nerve cord and body wall. Blood glands are present on the 4th, 5th and 6th segments. The blood glands produce blood cells and haemoglobin. Blood cells are phagocytic in nature. Exchange of gases occurs through moist body surface into the blood stream.
Nephridia are the excretory organs in earthworm. Nephridium is composed of coiled tubules. There are three types of nephridia, viz. septal, integumentary and pharyngeal nephridia.
Septal Nephridia: These are present on both sides of intersegmental septa of segment 15 to the last. The septal nephridia open into intestine.
Integumentary Nephridia: These are attached to the lining of the body wall of segment 3 to the last. The integumentary nephridia open on the body surface.
Pharyngeal Nephridia: These are present as paired tufts in the 4th, 5th and 6th segments.
A neprhidium is a funnel-like structure. It collects excess fluid from coelomic chamber. The tube at the end of the funnel carries the wastes into the digestive tube, through a pore on the surface in the body wall.
The nervous system is composed of a ventral pair of nerve cord. Ganglia are arranged in each segment on this paired nerve cord. The nerve cord in the anterior region (3rd and 4th segments) bifurcates and encircles the pharynx to join the cerebral ganglia. This forms a dorsal nerve ring.
Sensory System: There is no eye in the earthworm. But light and touch sensitive receptor cells are present. Chemoreceptors are also present. The sense receptors are present on the anterior part of the body.
Earthworm is hermaphrodite, which means both male and female sex organs are present in the same animal.
Male Reproductive System
There are two pairs of testes present in the 10th and 11th segments. The vasa deferentia run up to the 18th segment, where they join the prostatic duct. Two pairs of accessory glands are present in the 17th and 19th segments. The common prostate and spermatic duct opens to the exterior by a pair of male genital pores. The male genital pores are present on the ventro-lateral side of the 18th segment. Four pairs of spermathecae are located in 6th to 9th segments. During copulation, spermatozoa are stored in the spermathecae.
Female Reproductive System
One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th segments. Ovarian funnels are present beneath the ovaries. The ovarian funnels continue into oviduct. They join together and open on the ventral side as a single median female genital pore on the 14th segment.
Fertilization & Development: During mating, a mutual exchange of sperms occurs between two worms. Mature sperms, egg cells and nutritive fluid are deposited in cocoons produced by the gland cells of clitellum. Cocoons are deposited in soil. Fertilization and development occur within the cocoons. After about 3 weeks, each cocoon produces two to twenty baby worms. The average number of baby worms from a cocoon is four. Development is direct.
Economic Importance: Earthworms are called Friends of Farmers. They burrow in the soil and make it porous. It helps in respiration and penetration of developing plant roots. Earthworms are also used as bait in fishing.