Earthworm – Characteristic Features
Earthworms are nocturnal animals. They lie in the burrows during the day and come out at night for food. Earthworms leave the burrow only during the rainy season when their burrows are flooded with water.
Earthworm External features
Lampito (Megascolex) mauritii is a common earthworm found in South India. The body is long, slender, cylindical and bliaterally symmetrical. It is about 8 to 21 cm long and 3 to 4 mm in thickness. The dorsal surface is dark purplish brown, and the ventral surface is paler in colour. It is marked by a series of segments. The segments are separated from one another by intersegmental grooves. The division is both external and internal. Inside the body, each cavity of the segment is separated from the next, by a thin partition called the septum. All the segments look alike. This kind of repetitive arrangement of the segments is called metamerism.
The mouth is found in the centre of the first segment of the body, called the peristomium. Overhanging the mouth is a small flap called the upperlip or prostomium. The last segment has the anus. It is called the pygidium. In mature worms, segments 14 to 17 may be found swollen with a glandular thickening of the skin called clitellum.
Tiny curved bristles called setae are found embedded in small pits of the body wall. These pits are called the setigerous pits. The setae are arranged around the body. They are made of chitin and have a swollen middle part and pointed curved ends. The setae resemble the mathematical symbol. They can be moved in any direction and extended or withdrawn by the action of muscles. They are used for locomotion.
External apertures :
(i). Dorsal pores : These are minute openings situated in the mid dorsal line in the intersegmental grooves commencing from the 10th segment. The coelom communicates to the exterior through these pores and keep the body surface moist and free from harmful micro organisms.
(ii). Spermathecal openings : Three pairs of openings are situated ventrolaterally in the intersegmental grooves between segments six and seven, seven and eight and eight and nine. These opening can be easily seen in mature worms.
(iii). Openings of oviduct : These are a pair of apertures lying close together on the ventral surface of the 14th segment.
(iv). Openings of Spermiduct : A pair of apertures are situated on the lowerside of the 18th segment.
(v). Nephridiopores : Numerous minute openings scattered on the body wall from 14th segment onwards.
Body wall :
The body wall of earthworm is thin soft and moist. It consists of the following layers arranged from outside.
Cuticle : It is a thin, transparent, iridescent layer secreted by the underlying epidermis.
Epidermis : It is in the form of a single layer of columnar cells. This layer contains gland cells and receptor cells.
Dermis : It is a very thin sheet of connective tissue forming a basement for the epithelial cells on the outside and muscles on the inside.
Muscles : The muscles are arranged in two layers, namely the outer circular and inner longitudinal.
Coelomic epithelium : It is the inner most layer of the body wall forming the lining of the body cavity.
Body Cavity :
A spacious body cavity called the coelom is seen between the alimentary canal and the body wall. It is divided into a series of compartments by the transverse partitions of connective tissue called the septa. The coelom is lined with the coelomic epithelium and filled with coelomic fluid. It is a colourless fluid with amoeboid coelomic corpuscles floating in it. The fluid oozes out through the dorsal pores. It keeps body surface moist as a condition quite essential for respiration. The coelomic cavity communicates to the exterior through reproductive and excretory apertures. The germ cells are budded off from the wall of this cavity.
Earthworms move about by contraction and expansion of its body wall. When the circular muscles of the body wall contract, the body becomes thin and elongated. This process results in the forward extension of the body. Then it fixes itself firmly to the ground with help of the body setae and mouth. Subsequently when the longitudinal muscles contract, the body becomes thick and shortened. As a result, the body is drawn forward towards the anterior end which is already fixed to the ground. Thus by a repeated process of alternate contraction and expansion of muscular body wall locomotion is effected.
Digestive System :
The digestive system runs as a straight tube from mouth to anus. The mouth is situated in the first segment. The mouth opens into the buccal cavity which occupies segments 1 and 2. The buccal cavity in turn leads into a thick muscular Pharynx. The pharynx occupies segments 3 and 4 and is surrounded by the pharyngeal glands. The oesophagus is a short narrow tube lying in 5th segment. It leads into the gizzard lying in the 6th segment. Its inner surface has a chitinous lining. The intestine is a large tube extending from the gizzard to the anus. The intestine upto the 14th segment is narrow and the remaining part is sacculated. The dorsal wall of the intestine is folded into the cavity as the typhlosole. This fold contains blood vessels. It increases the absorptive area of the intestine. The inner epithelium consits of columnar cells and glandular cells.
The earthworm feeds on decaying organic materials contained in the soil. It takes the soil into its alimentary canal where the organic matter is digested and absorbed. The unwanted soil particles are sent out as worm casts.
Circulatory System :
In the body of earthworm there are two median longitudinal vessels. The dorsal longitudinal vessel runs above the alimentary canal. The ventral longitudinal vessel runs below the alimentary canal. The dorsal vessel is contractile and blood flows forwards in it. There are paired valves inside this vessel which prevent the backward flow of the blood. The ventral vessel is non contractile and blood flows backwards in it. The ventral vessel has no valves. In the anterior part of the body the dorsal vessel is connected with the ventral vessel by eight pairs of commissural vessels or the lateral hearts lying in the segments 6 to 13. These vessels run on either side of the alimentary canal and pump blood from the dorsal vessel to the ventral vessel.
Now that you have good understanding of Earthworms of life systems why don’t you take quiz about it here – Phylum Coelenterata Quiz