Morphology and anatomy of earthworm
- Earthworm is a reddish brown terrestrial invertebrate that inhabits the upper layer of the moist soil.
- The faecal deposits of earthworm are known as worm castings.
- The common Indian earthworms are Pheretima and
- Earthworms are known as ‘friends of farmers’ because they make burrows in the soil and make it porous.
- The process of increasing fertility of soil by the earthworms is called
- Earthworms have long cylindrical body divided into 100-120 similar short segments known as metameres .
- The dorsal surface of the body is marked by a dark median mid dorsal line along the longitudinal axis of the body and the ventral surface is distinguished by the presence of genital openings.
- Anterior end consists of the mouth and the prostomium, covering the mouth.
- The first body segment is called the peristomium, which contains the mouth.
- In a mature worm, segments 14-16 are covered by a prominent dark band of glandular tissue called clitellum, thus dividing the body into preclitellar, clitellar and postclitellar segments.
- Four pairs of spermathecal apertures and a pair of male genital pores are situated on the 5th -9th segments and 18th segment, respectively.
- A single female genital pore is present in the mid-ventral line of 14th segment.
- Numerous minute pores called nephridiopores open on the surface of the body.
- Except the first, last and clitellum, there are rows of S-shaped setae in each segment, help in locomotion.
Fig. Morphology of earthworm
- The body wall of the earthworm is covered externally by the epidermis, two muscle layers which are circular and longitudinal respectively and an innermost coelomic epithelium.
- Single layer of columnar epithelial cells present in epidermis contain secretory gland cells.
- The alimentary canal is a straight tube and runs between first to last segment of the body.
- A terminal mouth opens into the buccal cavity (1-3 segments) which leads into muscular pharynx.
- A small narrow tube, oesophagus (5-7 segments), continues into a muscular gizzard (8-9 segments), helps in grinding the soil particles and decaying leaves, etc.
- The stomach extends from 9-14 segments.
- Calciferous glands, present in the stomach, neutralise the humic acid present in humus.
- Intestine starts from the 15th segment onwards and continues till the last segment.
- A pair of short and conical intestinal caecae project from the intestine on the 26th segment.
- Typhlosole is the internal median fold of dorsal wall in the between 26-35 segments.
- The alimentary canal opens to the exterior by a small rounded aperture called anus.
- Pheretima exhibits a closed type of blood vascular system, thus blood is confined to the heart and blood vessels.
- Blood glands are present on the 4th, 5th and 6th segments which produce blood cells and haemoglobin.
- Respiratory exchange occurs through moist body surface into their blood stream.
- The excretory organs occur as segmentally arranged coiled tubules called nephridia.
- Nephridia is of three types-
- septal nephridia- present on both the sides of intersegmental septa of segment 15 to the last that open into intestine
- integumentary nephridia- attached to lining of the body wall of segment 3 to the last that open on the body surface.
- pharyngeal nephridia- present as three paired tufts in the 4th, 5th and 6th segments.
- A nephridium starts out as a funnel connects with a tubular part of the nephridium.
- Nervous system is basically represented by ganglia arranged segment wise on the ventral paired nerve cord.
- The nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the pharynx and joins the cerebral ganglia dorsally to form a nerve ring.
- Worms have specialized chemoreceptors and touch sensitive organs.
- Earthworm is hermaphrodite, possessing two pairs of testes present in the 10th and 11th segments, vasa deferentia run up to the 18th segment and two pairs of accessory glands in the 17th and 19th segments.
- Four pairs of spermathecae are located in 6th-9th segments.
- One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th segments.
- Ovarian funnels are present beneath the ovaries which continue into oviduct
- Earthworm mate juxtaposing opposite gonadal openings exchanging packets of sperms called
- Mature sperm and egg cells are nutritive fluid deposited in cocoons produced by the gland cells of clitellum.
- Fertilization and development occur within the cocoons.
- Each cocoon produces two to twenty baby worms with an average of four and the development is direct.
Fig. Anatomy of earthworm