In this article we will discuss about:- 1. Digestive System of Earthworm 2. Respiratory System of Earthworm 3. Circulatory System 4. Excretory System 5. Nervous System 6. Sense Organs 7. Reproductive System.
Digestive System of Earthworm:
The digestive system of earthworm consists of the alimentary canal and the digestive glands. The alimentary canal is a straight tube running throughout the length of the body from the mouth to the anus.
It consists of the following parts:
(a) Mouth:
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It is terminal crescentic aperture in the peristomium and overhung by prostomium.
(b) Buccal Cavity:
The mouth leads into a short thin walled tube called buccal cavity or buccal chamber. It extends from 1st to 3rd segment.
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(c) Pharynx:
It is a small pear shaped sac which is situated in the 4th segment. On the roof of the pharynx, there is a muscular digestive gland called pharyngeal bulb which contains a number of unicellular salivary glands in it. The lateral walls extending from the pharyngeal bulb divides the pharynx into two chambers – the dorsal salivary chamber and the ventral conducting chamber. The pharynx is attached to the body wall by a set of radial dilator muscles which dilates the pharynx during ingestion.
(d) Oesophagus:
Oesophagus also called gullet is a short, narrow, thin walled tube extends from 5th to 7th segment. The wall of the oesophagus is internally folded and it does not contain any glands.
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(e) Gizzard:
The oesophagus leads into a hard, oval, thick walled highly muscular sac called gizzard. It is situated in 8th and 9th segments. The gizzard is lined internally by the cuticle and it is the main grinding organ of the alimentary canal. Thus, it is mastigatory in function.
(f) Stomach:
The gizzard is followed by a narrow, thin walled tubular stomach which extends from 9th to 14th segment. The anterior and posterior openings of the stomach are sphinctered. The wall of the stomach is less muscular but highly glandular and vascular. Internally its wall is transversely folded. The glandular cells of the epithelial lining of the stomach secrete a proteolytic enzyme and the calciferous glands neutralize the humic acid of the soil. Excess calcium is eliminated as calcite.
(g) Intestine:
The stomach leads into the intestine. The intestine is a long, wide, thin-walled tubular part which extends from 15th segment up to the anus. The intestine is lined by ciliated and glandular cells. The intestinal lining forms folds called villi. One of these villi is comparatively large and is called typhlosole.
The typhlosole which hangs in the lumen of the intestine mid- dorsally runs from 26th segment up to the last except 24 or 25 segments in front of the anus. The presence of typhlosole divides the intestine into three parts – pre-typhlosolar part, typhlosolar part and post-typhlosolar part. Out of these 3 regions, both pre-typhlosolar and typhlosolar parts are digestive as well as absorptive while the post typhlosolar part corresponds to rectum.
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The first part of the intestine starting from 15th segment up to 26th segment constitutes the pre-typhlosolar region. There is no typhlosole in this region. The intestinal villi are highly vascular. In the 26th segment, the intestine gives off two short conical lateral outgrowths called intestinal caeca. These intestinal caeca extend anteriorly up to 22nd or 23rd segment.
These caeca are considered as digestive glands which secrete some amylase like enzyme.
The typhlosolar region is the second part of the intestine which extends from 27th segment up to the last 24 or 25 segments. This region is characterized by the presence of an internal median fold of the dorsal wall of the intestine called typhlosole. The epithelial lining of typhlosole comprise mainly of glandular cells which secrete a variety of digestive enzymes such as protease, lipase, amylase, chitinase and cellulase. The typhlosole also increases the absorptive surface of the intestine. Since the process of digestion gets completed in this region, it is considered as the major site of absorption.
The intestine in the last 24 or 25 segments in front of the anus has no typhlosole and this region is called post-typhlosolar region or the rectum. This region is thin walled, less vascularised and without villi and glandular cells. It contains small pellets of soil containing undigestible residue of food. These pellets are egested out as faeces through anus which form a heap of worm castings at the opening of the burrows.
(h) Anus:
The alimentary canal ends in anus at the terminal end if the centre of the last segment of the body.
Respiratory System of Earthworm:
Earthworm lacks specialized respiratory organs. The exchange of respiratory gases takes place through the body surface, Thus earthworms show cutaneous respiration. Therefore the skin is well adapted for this purpose. It is thin, moist and highly vascular.
The skin is often kept moist by the mucus secreted by the mucous gland cells of the epidermis, the coelomic fluid oozing out through the dorsal pores and the moisture of the soil. The oxygen which dissolves in these fluids diffuses into the blood through the skin. At the same time the carbon dioxide diffuses out from the blood to the air through the skin.
Circulatory System of Earthworm:
The circulatory or blood vascular system of earthworm is of closed type.
It consists of the following:
i. Blood:
The blood of earthworms is coloured red because of the presence of red respiratory pigment called haemoglobin or erythrocruorin. But unlike the vertebrates, the haemoglobin is found dissolved in the plasma. The plasma also contains colourless cells which are physiologically similar to the leucocytes of the vertebrates.
ii. Blood Vessels:
The blood vessels of Pheretima posthuma can be divided into three categories:
A. Longitudinal Blood Vessels:
These blood vessels run lengthwise in the body.
There are altogether five longitudinal blood vessels. They are:
(a) Dorsal Vessel:
It is the largest blood vessel of the body and runs mid-dorsally above the alimentary canal from one end of body to the other. It has thick, muscular and contractile wall and is provided with a pair of valves in front of the septum in each segment. This blood vessel is in fact considered as dorsal tubular heart. Blood flows in it from behind to forwards. The dorsal vessel is visible from outside as a dark line through the thin and semi-transparent body wall.
The dorsal vessel behind 13th segment acts as a collecting vessel, receiving blood through two pairs of dorso-intestinal vessels from the intestine and a pair of commissural vessels from sub neural vessel in each segment.
Anterior to 13th segment, the dorsal vessel acts as a distributing vessel, supplying blood to the anterior region of alimentary canal and to ventral vessel through the so called hearts. The dorsal vessel which extends upto cerebral ganglia trifurcates and these three branches are distributed over the pharyngeal bulb and roof of the buccal chamber. From the dorsal vessel pair of branches also arise in each of the 3rd, 4th, 5th, 6th and 8th segments and supply the blood to the pharynx, oesophagus, gizzard and stomach.
(b) Ventral Vessel:
It is a long vessel that runs mid-ventrally below the alimentary canal and above the nerve cord from the second segment up to the last segment of the body. It is a thin, non-muscular and non-contractile vessel and valves are altogether absent in it. Blood flows backward in it.
Ventral vessel is mainly a distributing vessel. In each segment it gives out a pair of ventro- tegumentary vessels which run upwards along the body wall and supply blood to the body wall, integumentary nephridia, septal nephridia, gonads, seminal vesicles and spermathecae. Behind 13th segment the ventral vessel in each segment gives off a median ventro-intestinal vessel to intestine.
(c) Lateral Oesophageal Vessels:
These are the two vessels which lie one on either ventro-lateral side of the alimentary canal, running from the second segment of body up to 13th segment. These act as collecting vessels and collect the blood from buccal cavity, pharynx, oesophagus and body wall. They also send the blood to the supra-oesophageal vessel through a pair of anterior loops lying in each of the segments 10 and 11 and through several ring vessels running along the wall of stomach. The remaining blood flows backward into sub-neural vessel.
(d) Sub-Neural Vessel:
It is a thin long vessel which runs mid-ventrally beneath the nerve cord from 14th segment up to the last segment. It is formed by the union of two lateral oesophageal vessels. The blood flows from anterior to the posterior end in it. It is mainly a collecting vessel and receives blood from ventral body wall and nerve cord in each segment through a pair of small branches. It pours blood via a pair of commissural in each segment into the dorsal vessel.
(e) Supra-Oesophageal Vessel:
It is a thin walled short collecting blood vessel lying mid-dorsally above the stomach between the segments 9 and 13. It is connected to lateral oesophageal vessel by two pairs of anterior loops and to ventral vessel by two pairs of latero-oesophageal hearts. The anterior loops are thin walled, non-pulsatile, non-muscular, loop like blood vessels without valves. They are situated one pair in each of the 10th and 11th segments. The supra-oesophageal vessel collects blood from gizzard, stomach and from lateral oesophageals and pumps it into ventral vessel through lateral oesophageal hearts.
B. Lateral or Transverse Blood Vessels:
All the longitudinal blood vessels of earthworm are interconnected with one another through a number of segmentally arranged lateral or transverse blood vessels. These include lateral hearts, latero-oesophageal hearts, anterior loops, ventrotegumentary vessels, commissural vessels, dorso-intestinals and ventro-intestinals.
C. Intestinal Plexuses:
The wall of the intestine contains two plexuses namely, the on the surface and internal plexus between the circular muscles and enteric epithelium. The external plexus receives the blood from the ventral blood vessel through ventro-intestinals and septo-intestinals. This plexus in turn passes the blood into the internal plexus. Internal plexus passes the blood along with the absorbed nutrients to dorsal blood vessel through dorso-intestinals.
iii. Hearts:
In earthworms, in each of the segments 7, 9, 12 and 13 a pair of large, thick, muscular and valvular vertical vessels are found. These are called hearts. Out of these, the lateral blood vessels which occur on 7th and 9th segments are called lateral hearts and these are the direct connections between dorsal and ventral blood vessels.
The lateral blood vessels of 12th and 13th segments are called lateral oesophageal hearts. These connect the dorsal and the supra oesophageal blood vessel with the ventral blood vessel. In both types of hearts the blood flows from dorsal vessel and supra oesophageal blood vessel towards the ventral blood vessel.
Although these blood vessels are called hearts, in a true sense they are not at all hearts because they do not show any kind of pulsatile activity which a normal heart shows. Both lateral and latero-oesophageal hearts have the valves that allow the blood to flow in one direction only, i.e., towards ventral vessel. Each lateral heart has four pairs of valves while each latero-oesophageal heart has three pairs of valves.
iv. Blood Glands:
In the 4th 5th and 6th segments lying above the pharyngeal mass there are several groups of small rounded follicles of red colour called blood glands. These glands consist of a mass of loose cells surrounded by a capsule with a syncytial wall. The blood glands are connected with pharyngeal nephridia and with salivary glands. These glands help in the manufacture of blood corpuscles and haemoglobin. These glands are also considered to be excretory in function.
Excretory System of Earthworm:
The excretory system of earthworms is made up of segmentally arranged microscopic slender coiled tubules called nephridia (or micronephridia). In Pheretima the nephridia are found in all segments except the first three.
Based on the location in the body, the nephridia are classified into three types:
i. Pharyngeal Nephridia:
These occur as paired tufts on either side of pharynx and oesophagus in the 4th, 5th and 6th segments. The pharyngeal nephridium is of closed type having no funnel or nephrostome. These nephridia discharge their excretory products into the alimentary canal and are therefore called enteronephric nephridia. The pharyngeal nephridia which are situated in the 4th and 5th segments open into the pharyngeal region while those of 6th segment open into the buccal cavity.
ii. Integumentary Nephridia:
The integumentary nephridia are found scattered all over the entire inner or parietal surface of the body wall except the first two segments. There are around 200-250 integumentary nephridia in each segment with the exception of clitellar segments where the number increases to more than 2000 in each segment, thus forming ‘forests of nephridia’. The integumentary nephridia are the smallest ‘v’ shaped nephridia without nephrostome hence closed type. They open on the body surface directly through minute pores called nephridiopores, thus they are exonephric.
iii. Septal Nephridia:
These are the largest nephridia of Pheretima. They occur on both the anterior and posterior surfaces of all the septa behind the segment 15, wherein each septum bears 4 rows of septal nephridia, 2 on its anterior surface and 2 on the posterior surface. Each row has about 20 to 25 nephridia, thus there are 80 to 100 nephridia on each septum.
The septal nephridium which is considered as the typical nephridium is formed of four parts -nephrostome, neck, body of nephridium and the terminal duct.
Nervous System of Earthworm:
The nervous system consists of the nerve ring and a ventral nerve cord.
The nerve ring is formed of paired cerebral or supra-pharyngeal ganglia, circumpharyngeal or peripharyngeal connectives and sub-pharyngeal ganglia. A pair of closely united pear-shaped cerebral ganglia forms the so called brain. It lies dorsally in the 3rd segment in the depression between the buccal cavity and the pharynx.
A pair of thick circum-pharyngeal connectives that arises from the brain laterally encircles the pharynx and meets the sub-pharyngeal ganglia beneath the pharynx in the 4th segment. Thus a complete nerve ring or brain ring is formed around the pharynx.
Arising from the sub-pharyngeal ganglia there is a double ventral nerve cord which runs up to the posterior end of the body mid-ventrally beneath the ventral vessel but above the sub-neural vessel. Although the ventral nerve cord is double, it appears to be single as the two cords are united with each other.
In each segment from 5th to the last segment the nerve cord bears a ganglion or swelling. Each segmental ganglion is formed by the fusion of a pair of ganglia, one belonging to each cord of the double ventral nerve cord.
Sense Organs of Earthworm:
In earthworms the sense organs are poorly developed and are formed of single or group of sensory cells called receptors.
They are of three types:
a. Epidermal Receptors:
The epidermal receptor consists of group of tall cells in the epidermis which results in the formation of elevation of the cuticle. Although epidermal receptors are found all over the epidermis in the body, they are more numerous in the lateral and ventral regions. The epidermal receptors are mainly tangoreceptors (tactile in function) but they also perceive thermal and chemical stimuli.
b. Buccal Receptors:
Buccal receptors are confined to the buccal cavity where they are found in large numbers. The buccal receptors consists of groups of tall cells which project beyond the epithelial cells. The buccal receptors help the animal to smell (i.e., olfactoreceptors) and taste food (i.e., gustatoreceptors).
c. Photo-Receptors:
Photoreceptors are confined only to the dorsal surface and are not found on the ventral surface. They are found more on the prostomium and first segment and their number gradually decreases from anterior to the posterior region of the body. The earthworm shows negative response to even very weak light. That is why they retreat into the burrows during day.
Reproductive System of Earthworm:
Earthworm is a hermaphrodite or bisexual or monoecious form. It has both male and female reproductive organs in the same individual. But it always shows cross-fertilization due to protandrous condition wherein the testes mature earlier than ovaries and self-fertilization is prevented. Pheretima shows only sexual reproduction.
Male Reproductive System:
The male reproductive system consists of:
(a) Testes:
There are two pairs of testes, one pair in each of 10th and 11th segments, on either side of the alimentary canal. Each testis which is a whitish lobed structure consists of 4 to 8 finger like lobules projecting from a compact base. Each lobule contains rounded cells called spermatogonia.
(b) Testes Sacs:
The testes of each segment are enclosed in thin walled, bilobed, wide yellowish fluid-filled sacs called testis sac. Thus, there are two testes sacs, one in the 10th and another in the 11th segment. They are situated beneath the stomach. Posterior testis sac is larger in size than the anterior testis sac. The testes sacs also enclose seminal funnels.
(c) Seminal Vesicles:
The seminal vesicles are the large glandular sacs situated in the 11th and 12th segments. They are two pairs in number. The testis sac of 10th segment communicates with the seminal vesicles of 11th segment and the testis sacs of 11th segment communicate with those of 12th segment.
The seminal vesicles help in the maturation and storage of sperms. The spermatogonia contained in the testes are released into the testes sacs; from there they enter the seminal vesicles. The mature sperms return back into the testes sacs and then enter the spermiducal funnels.
(d) Seminal Funnels or Spermiducal Funnels or Sperm Rosettes:
The testes sacs on their inner side also enclose ciliated seminal funnels which receive the sperms from seminal vesicles.
(e) Vasa Deferentia:
Each seminal funnel leads into a long tubular duct called vas deferens. The two vasa deferentia of the same side run close together upto 18th segment and open on the ventral side of 18th segment along with the prostatic duct by male genital aperture.
(f) Prostate Glands:
These are a pair of whitish lobated glands situated between the 16th or 17th and 20th or 21st segments one on each side of the intestine. In the 18th segment, a short muscular prostatic duct arises from each prostate gland which joins the vasa deferentia of that side and forms a common spermatic duct. The latter opens through the male genital aperture.
(g) Accessory Glands:
Situated on the ventral side of 17th and 19th segments there are two pairs of accessory glands. These glands discharge their secretions outside the body through two pairs of genital papillae which are situated ventrally on the 17th and 19th segments. This slimy secretion helps in keeping the two worms together during copulation.
Female Reproductive System:
The female reproductive system consists of:
(a) Ovaries:
A pair of ovaries is found below the alimentary canal, one on each side of the ventral nerve cord, attached to the posterior surface of the intersegmental septum between 12th and 13th segments. Each ovary is a white mass and formed of finger shaped processes called ovarioles. Inside the ovarioles the ova are arranged in a linear order with mature ova at the distal part and immature in the proximal part. Matured ova detach from the ovary and are shed into the coelomic cavity of the 13th segment from where they are taken by the oviducts.
(b) Oviducts:
Below each ovary in the 13th segment there is a ciliated funnel called ovarian funnel or oviducal funnel. Each oviducal funnel leads into a short tubular ciliated oviduct. The two oviducts pass through the septum of 13th segment and meet together below the nerve cord and open on the mid-ventral side of the 14th segment by a single female genital aperture. The mature ova which are released from the ovaries and received by the oviducal funnels pass through the oviducts and go out through the female genital aperture.
(c) Spermathecae:
There are four pairs of spermathecae, one pair in each of 6th, 7th, 8th and 9th segments. Each spermatheca is a flask shaped structure consisting of a pear shaped ampulla and a short neck, the latter gives off a pouch like caecum or diverticulum. The short duct or neck of the spermatheca opens outside by the spermathecal aperture. The four pairs of spermathecal apertures are situated ventro-laterally in the intersegmental grooves of 5/6, 6/7, 7/8 and 8/9 segments. The spermathecae receive and store the sperms during copulation.
Copulation and Fertilization in Earthworms:
Earthworms breed during rainy season. During rainy days, earthworms come out of their burrows at night and copulate. During copulation two mature earthworms attach to each other by their ventral sides with the anterior ends in opposite directions in such a way that the male genital apertures of one worm are fitted against the 1st pair of spermathecal pores of the other. This attachment is helped by the genital papillae of both worms. For some time the two animals remain in this position.
Later both the worms slightly slide backwards so that the male genital apertures now fit into the 2nd pair of spermathecal pores and the process continues with respect to 3rd and 4th pairs of spermathecal pores. Thus all the four pairs of spermathecae get filled with sperms of other worm. The copulation lasts for about an hour, after which the earthworms separate.
After the separation of two individuals, the gland cells of the clitellum produce a chitinous secretion which on exposure to air gets hardened to form an elastic girdle around the clitellum. When the girdle is fully formed, the animal wriggles backward to come out of it. As the girdle passes over the female genital aperture, it receives the ova. When the girdle passes over the spermathecae, it receives sperms collected from the other individual.
As soon as the animal completely comes out of the girdle, the two open ends of the girdle get closed and the girdle is now called cocoon. Such cocoons are laid on the ground hidden under moist soil. The eggs in the cocoon get fertilized by the sperms. So in earthworms the fertilization is external and is always referred to as reciprocal cross fertilization.