In this article we will discuss about:- 1. Habitat of Earthworm 2. Alimentary System of Earthworm 3. Circulatory System 4. Respiratory System 5. Nervous System 6. Excretory System 7. Reproductive System 8. Life-History.
Contents:
- Habitat of Earthworm
- Alimentary System of Earthworm
- Circulatory System of Earthworm
- Respiratory System of Earthworm
- Nervous System of Earthworm
- Excretory System of Earthworm
- Reproductive System in Earthworm
- Life-History of Earthworm
1. Habitat of Earthworm:
Earthworms are soft-bodied creatures which live under damp soil within burrows which may extend two (5 cms) or even three feet (7·5 cms) underground. They come out of their holes at night for feeding and return back at daybreak.
During rains, when their burrows are flooded, the animals crawl about in damp places, hiding amongst heaps of dried leaves. They bore their way through the soft earth, eating the soil as they go in. The swallowed earth passes through the alimentary canal and deposited as small pellets of mud lying at the opening of the burrow on the surface of the soil. These pellets are popularly known as ‘worm-casts’.
In doing this the earthworm makes the soil more porous and their castings supply a fine top-dressing for the surface. It is for this reason that Charles Darwin called them the natural tillers of the soil. The earthworms, therefore, are real friends of the farmers.
2. Alimentary System of Earthworm:
The alimentary canal runs straight from mouth to anus. The mouth leads into a thin-walled buccal chamber which is continuous with a wide and muscular pharynx in the third segment. The pharyngeal wall is thickened dorsally to produce glandular mass called pharyngeal bulb.
The pharynx is followed by the oesophagus which runs back from the fourth to the fourteenth segment. A part of the oesophagus dilates into an oval gizzard in the eighth segment. This is a thick- walled muscular organ lined internally
The next portion system of earthworm of the alimentary canal is the thin- walled intestine which extends from the fourteenth to the ninety- fifth segment. The dorsal wall of the intestine is in-folded to form an internal ridge or typhlosole, which increases the digestive surface and hangs like a curtain into the lumen of the gut.
The intestine bears a pair of small, finger-like, blind pouches or caeca in the 26th segment. The rest of the alimentary canal, lying in the last twenty-five segments of the body, is known as the rectum. It is devoid of typhlosole and opens by the anus at the posterior end of the last segment.
Feeding and Digestion:
Food of earthworm consists mainly of organic matter in the soil, such as bits of leaf and particles of animal matter. Soil may be taken at any time but the leaves, etc., must be secured at night, when the worm crawls out into the air.
The food is sucked into the mouth by the alternate expansion and contraction of the muscular pharynx. It is moistened by saliva which is secreted from glands of the pharyngeal bulb and passed into the oesophagus.
The secretion of oesophageal glands neutralises the organic acids of the soil. The food is then ground up in the gizzard and broken into minute fragments by repeated squeezing against the hard internal cuticle. A series of peristaltic waves drive the food backwards along the alimentary tube.
Meanwhile, digestion is effected by enzymes. Ultimately, the food is passed into the intestine where most of the digestion and absorption take places. The ‘castings’ are voided through the anus in the form of small rounded pellets or balls, each distinct from the others.
3. Circulatory System of Earthworm:
The circulatory system of earthworm is a complicated system of closed tubes. The blood circulating in these vessels consists of a liquid plasma and a number of colourless amoeboid cells called corpuscles.
The red colour of the blood is due to the pigment haemoglobin which remains dissolved in the plasma. There are no red blood corpuscles. The blood is propelled by a sort of peristaltic contraction of the muscle fibres in the wall of the vessels. Some of the vessels contain valves to prevent leakage and backflow.
A number of vessels are large and centrally located. They run along the length of the body, and give off branches which ultimately break up into capillaries. There is no true heart; but there are four pairs of dilated pulsatile loops, one pair in each of the 7th, 9th, 12th and 13th segments. These are the so-called lateral hearts, because they possess the power of rhythmic contraction.
There are three main longitudinal vessels, running parallel to one another along the whole length of the body.
(1) There is a large dorsal blood vessel in the mid-dorsal line between the gut and the body wall. It extends from the hinder end of the body to the pharynx, where it breaks up into branches which ramify over and supply blood to that organ.
The muscular wall of the dorsal vessel contracts and expands rhythmically, driving the blood towards the anterior end. This vessel is provided with valves in its interior which allow blood to pass forwards, but prevent back- flow.
(2) The ventral vessel runs along the mid-ventral line just beneath the gut. It contains no valves and the blood in it flows backwards.
(3) A slender sub-neural vessel runs along the mid- ventral line just beneath the ventral nerve cord. In the 13th segment of the body the sub-neural vessel bifurcates to form the two lateral oesophageal vessels, running along the sides of the oesophagus up to the anterior end of the body (Fig. 73).
It is to be noted that the dorsal vessel is a collecting tube receiving blood through smaller branches. The ventral and the sub-neural vessels, on the other hand, are distributing channels supplying branches to various parts and organs.
Besides the three main longitudinal trunks, there is a slender supra-intestinal vessel lying dorsal to the oesophagus but placed ventral to the dorsal blood vessel. It extends from the 9th to the 13th segment. In the 10th and 11th segments, the supra-intestinal vessel is connected to each of the two lateral oesophageal vessels by slender loops.
The four pairs of lateral hearts, one pair in each of the segments 7th, 9th, 12th and 13th, connect the dorsal vessel with the ventral vessel. They are provided with valves so as to direct the blood to flow downwards, that is from the dorsal to the ventral vessels. The two posterior pairs are joined to the supra-intestinal vessel.
In each segment in the intestinal region, there are two ring like transverse vessels surrounding the intestine , and a commissural vessel run along the posterior face of the septum in front. The dorsal vessel receives, on each side, in each segment, two dorso-intestinal vessels from the ring-like transverse vessels. The commissural vessel is connected by a branch with the sub-neural vessel lying beneath the nerve cord.
In each segment, the ventral vessel gives off a pair of ventro tegumentary vessels to supply the body wall and the nephridia of the segment. A median ventro-intestinal branch is supplied to the intestine from the ventral vessel, in each segment.
4. Respiratory System of Earthworm:
The earth worm has no separate respiratory organ. It receives oxygen and gets rid of carbon dioxide through its moist skin. The highly vascular epidermis is a permeable membrane through which O2—CO2 exchange is possible. Respiration takes place through the whole surface of the body.
5. Nervous System of Earthworm:
The central nervous system consists of a nerve ring encircling the pharynx and a ventral nerve cord with ganglionic swellings.
The nerve ring is composed of:
(1) A pair of suprapharyngeal ganglia or brain lying on the dorsal surface of the pharynx in the third segment;
(2) A sub-pharyngeal ganglionic mass lying ventral to the pharynx in the fourth segment, and
(3) Two peripharyngeal connectives, one on each side of the pharynx, joining the suprapharyngeal ganglia with the sub-pharyngeal ganglionic mass.
The ventral nerve cord runs straight backwards from the sub-pharyngeal ganglionic mass and extends along the mid-ventral line up to the posterior end of the body. The ventral nerve cord bears a ganglion in each segment behind the fourth.
Each segmental ganglion really consists of two ganglia fused together. The cord itself is double, the two longitudinal strands being fused together beyond recognition (Fig. 75). The major part of the nervous system lies ventral to the gut.
Peripheral nerves pass out from the various ganglia to supply different parts of the body. The nerves are of two kinds—afferent or sensory and efferent or motor.
Receptor Organs:
The sensitiveness of earthworm to touch, smell, light and other stimuli is due to the presence of a number of epidermal receptor organs. These are groups of specialised epidermal cells connected with the nervous system by afferent nerves. They occur more at the anterior and posterior ends than in the middle part of the body.
6. Excretory System of Earthworm:
The excretory matter is carried outside by a number of coiled tubes, called nephridia, which are present in every segment of the body excepting the first two. Excreta are probably stored up as granules within the yellow coloured chloragogen cells situated superficially round the intestine. The granules are discharged into the coelom, from where they are either collected by the nephridia or thrown out by the ‘dorsal pores.
The nephridia are networks of fine tubules. Some nephridia open into the coelom by funnel-shaped nephrostomes. Some open to the exterior through minute openings or nephridiopores scattered throughout the skin; and some communicate by ducts with the alimentary canal.
The nephridia are of three kinds:
(1) Integumentary:
The integumentary nephridia are extensively distributed over the inner surface of the body wall. These are small U-shaped tubes, about 200 of which are present in each segment of the body. They have no nephrostomes but open to the exterior individually, each by its own nephridiopore.
(2) Pharyngeal:
The pharyngeal nephridia occur in tufts near the pharynx in the 4th, 5th and the 6th segments. They have no nephrostomes or coelomic funnels. There are no nephridiopores. The terminal ends of the nephridia of a segment join to form a common duct which opens into the lumen of the pharynx.
(3) Septal:
The septal nephridia are attached to all inter-segmental septa behind the 15th segment. About 50 of them are found on either surface of each septum, hanging freely into the body cavity. A septal nephridium is the largest and represents the typical form. It consists of a funnel-shaped nephrostome opening into the coelom. The funnel is connected by a small tube with the main body of the nephridium.
The latter consists of a long spirally twisted loop and a terminal nephridial duct which leads into the septal excretory canal. There is one pair of septal excretory canals on each septum and they communicate with a pair of supra-intestinal excretory- ducts which run longitudinally in the mid-dorsal line of the intestine below the dorsal blood vessel.
Each supra-intestinal duct receives all the septal excretory canals of the same side and open segmentally by short vertical tubes into the lumen of the intestine.
All the nephridia are bathed in coelomic fluid and very richly supplied by fine blood capillaries. They are composed of glandular cells and partly lined by ciliated epithelium the cilia of which work towards the excretory canals.
Waste products are selected out by the nephridial cells from the blood and the body fluid. These are gathered within the nephridial ducts. Thence the waste products are conveyed to the gut and eliminated through the anus along with the faeces.
The waste products collected by the integumentary nephridia, however, are discharged directly to the exterior through nephridiopores situated on the outer surface of the body. The septal and the pharyngeal nephridia are said to be enter nephric because they open into the gut. The integumentary nephridia belong to the exonephric type because they open outside by nephridiopores.
7. Reproductive System in Earthworm:
The earthworm is monoecious or hermaphrodite. Male and female reproductive organs occur in the same individual. The male reproductive organs consist of two pairs of testes lying beneath the alimentary canal in the 10th and 11th segments.
Each testis is a small whitish body which manufactures sperm cells. A ciliated seminal funnel lies behind each testis, so that one pair of funnels occur in the 10th and another pair in the 11th segment.
Each funnel is continued behind as a slender vas deferens. The two vasa deferentia of one side run together along the ventral body wall up to the 18th segment where they join with the prostatic duct of the same side.
In the 10th and 11th segments, the testes and the seminal funnels of each side are enclosed in a hollow testis-sac. The cavities of the two testis-sacs in the 10th segment, as well as of the two in the 11th segment, are continuous. There is a pair of horn-like seminal vesicles in the 11th and 12th segments which open into the testis-sacs.
The testis-sacs of the 10th segment are continuous with the seminal vesicles of the 11th segment, and the testis-sacs of the 11th segment are continuous with the seminal vesicles of the 12th segment. The spermatozoa are matured and nourished within the cavities of the seminal vesicles.
A pair of white elongated bodies called prostate glands extend from the 16th to the 21st segment of the body. They are irregular in shape and lie one on each side of the intestine.
A prostatic duct comes out from each prostate and is at once joined by the two vasa deferentia of the same side to form a horse-shoe- shaped spermatic duct. The two spermatic ducts open ventro- laterally on the body wall through the male gonopores, situated in the 18th segment.
There are two pairs of accessory glands on the ventral body wall, one pair in the 17th and the other in the 19th segment. They lie close to the inner side of the prostate glands and open externally as genital papillae in front of and behind the male gonopores. The exact function of the prostates and the accessory glands are not known.
The female reproductive organs consist of a pair of earthworm ovaries, one on each side of the nerve cord, and attached to the posterior surface of the septum between the 12th and 13th segments. Each ovary is a mass of elongated white threads containing the eggs or ova.
An oviduct, with a funnel-shaped opening, is placed behind each ovary. The short oviducts pass backwards and join with one another in the mid-ventral line of the 14th segment to communicate with the female gonopore which is situated there.
There are four pairs of sperm-sacs or spermathecae, one pair in each of the 6th, 7th, 8th and 9th segments. Each spermatheca is a small white sac which stores spermatozoa received from another worm during copulation. They communicate with the exterior through the spermathecal openings, situated ventro-laterally in the inter-segmental grooves of the corresponding segments.
8. Life-History of Earthworm:
Mating takes place during summer within underground burrows. Two worms come together in a head-to-tail position, so that the spermathecal segments of the one are pressed against the clitellar segments of the other. A mutual exchange of spermatozoa takes place, after which the worms separate.
A membranous girdle is formed by the secretion of the clitellar glands surrounding the clitellar segments. This hardens slowly on exposure to air, and the worm endeavors to pull itself out of it. As the girdle is pushed over the female gonopore and the spermathecal openings, it receives ova and sperms.
The two ends of the girdle are sealed as soon as it is cast off by the worm and the resulting cocoon is deposited in damp soil. Cocoon formation takes place during the summer months. The ova are fertilized inside the cocoon. Development is direct. There is no larval stage.
As a rule, one embryo hatches out of each cocoon. A cocoon is a small chitinous structure, more or less spherical in shape and is of light yellow colour. It is swollen in appearance and is filled with albumen which nourishes the developing embryo.