- The body wall of earthworm has an external covering of a thin, flexible cuticle.
- The cuticle has many perforations, though which epidermal glands release their secretions.
- The epidermis consists of
- Columnar cells (supporting cells)
- Glandular cells (which secrete mucus)
- Sensory or receptacle cells (receives various types of stimuli)
- Basal cells (present between columnar cells)
- Setal sacs are also present in the epidermis.
- There are two layers of muscles beneath the epidermal layer.
- The outer layer is composed of circular muscles.
- The inner layer is made of longitudinal muscles.
- The innermost layer of body wall is the coelomic epithelium.
- The digestive system is complete with mouth and an anal opening.
- Thus, the body plan is tube-within-tube plan.
- The mouth opens into the buccal cavity (segments 1 and 2).
- The buccal cavity leads into a thick muscular pharynx which occupies the 3rd and 4th segments. It is surrounded by pharyngeal glands.
- The pharynx follows a small narrow tube called oesophagus (5-7 segments), which continues into a muscular gizzard (8-9 segments).
- Gizzard helps in grinding the food which is mostly soil particles, decaying leaves and other organic matter.
- The stomach extends from 9-14 segments.
- Stomach contains calciferous glands, which can neutralise the humic acid present in humus.
- Intestine extends from 14nth segment to last segment.
- In the 26th segment, a pair of short and conical intestinal caecae projects from the intestine.
- The end parts of intestine possess several internal median folds of dorsal wall called typhlosole. This increases the effective area of absorption in the intestine.
- The alimentary canal ends in a small rounded aperture called anus.
- The excretory organs are coiled tubules called nephridia.
- They are present in each segment (except 1 and 2).
- There are 3 types of nephridia:
- Pharyngeal nephridia: They are found in pairs in the fourth fifth and sixth segments.
- Integumentary nephridia: These are found attached to the inner surface of the body wall from the third segment the last segment.
- Septal nephridia: These are found from segment 15 to the last segment. They open into the intestine.
- Functions of nephridia: Regulation of volume and composition of the body fluids.
- Structure of nephridia:
- Initial part of nephridium is a funnel shaped structure called nephrostome. It collects excess fluids from the coelomic chamber.
- The funnel leads to a tubular part called nephric tube which delivers the collected wastes into the digestive tube.
- The wastes are expelled from the tubes through an external opening called nephridiopore.
- The Nervous system in earthworm consists of
- A cerebral ganglion (circumpharyngeal nerve ring)
- A double ventral nerve cord (extending from the fourth segment to the last segment)
- The nerve cord bears segmental ganglia from which paired lateral nerves arise.
- The nerve cord code in the anterior region (third and fourth segments) bifurcates, encircles the pharynx laterally enjoins the cerebral ganglia dorsally to form a nerve ring.
- Earthworms don’t have specialised breathing devices.
- Gas exchange occurs between the bloodstream and the moist body surface.
- Blood vessels absorb the oxygen and carry it to the body parts.
Earthworm has closed type of blood vascular system or circulatory system. It consists of blood vessels, capillaries and a heart.
- The blood is confined to the heart and blood vessels (since closed type of circulatory system is present).
- Contractions of heart keep blood circulating in one direction.
- Blood glands are present on the 4th 5th and 6th segments.
- The blood glands produce blood cells and haemoglobin which is dissolved in the plasma.
- Blood cells are usually phagocytic in nature.
- The Sensory system in earthworm is composed of light, touch and chemoreceptors.
- They do not have eyes. Instead, they have light receptor cells (photo receptors) which can distinguish lights of different intensity.
- Receptor cells present in the epidermis (touch / epidermal receptors) helps to feel the vibrations on the ground.
- They possess specialised chemoreceptors or taste receptors (buccal receptors) that respond to chemical stimuli.
- All these sensory organs are present on the anterior part of the body.
- Earthworms are called ‘friends of farmers’ because when they dig burrows in the soil, make the soil porous and soft.
- This in turn health helps the plants penetrate deeper into the soil and also aids in their respiration.
- In addition earthworms convert the dead organic matter in the soil into humus which increases the fertility of the soil.
- Earthworms are commercially used in the production of vermicompost. It is also one of the commonly used baits in fishing.