An Example of Phylum Annelida: Earthworm

In this article we will discuss about Earthworm:- 1. Habit and Habitat of Earthworm 2. External Structures of Earthworm 3. Locomotor Organs 4. Body Wall 5. Locomotion 6. Digestive System 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System 11. Reproductive System.

Contents:

1. Habit and Habitat of Earthworm
2. External Structures of Earthworm
3. Locomotor Organs of Earthworm
4. Body Wall of Earthworm
5. Locomotion of Earthworm
6. Digestive System of Earthworm
7. Respiratory System of Earthworm
8. Circulatory System of Earthworm
9. Excretory System of Earthworm
10. Nervous System of Earthworm
11. Reproductive System of Earthworm

1. Habit and Habitat of Earthworm:

The most common earthworm of our country is known as Pheretima posthuma. It is a member of the class Oligochaeta. Pheretima usually inhabits the moist soil. During day­time they live in underground burrows and at night and rainy season they come out of their shelters. The castings of earthworms, which are small rounded pellets or balls, lie at the openings of the burrows.

These cast­ings are formed when the animals go deep into the hard and closely packed soil by ‘eating’ earth. The eaten-up earth passes through the body and is deposited as ‘cast­ings’. While burrowing, the earthworms make the soil loose and porous. The body wastes of earthworm increase soil fertility. So the earthworms are called the natural tillers of land.

2. External Structures of Earthworm:

The body is elongated, narrow and cylin­drical (Fig. 17.14). The thickness of the body is more or less uniform. The anterior end is more pointed than the posterior end.

The dorsal side of the body is brown in colour and can be readily distinguished from the ventral side which is brightly coloured. The impression of the dorsal blood vessel can be seen on the dorsal side as a dark median line extending throughout the length of the body. A full-grown worm measures about 20 cm in length.

The body of the animal is made up of distinct segments or metameres separated from each other by inter-segmental grooves. The numbers of segments comprising the body are about 100-120. This external seg­mentation corresponds to internal segmenta­tion. The segments 14-16 from the anterior end are encased in a thick glandular tissue sheet, called the clitellum (saddle) or cingulum (belt).

Considering the clitellum as the index, the body may be divided into three regions, namely the pre-clitellar, clitellar and post-clitellar regions. Some of the ante­rior segments bear superficial furrows so that these segments appear to be subdivided. These are merely external subdivisions.

There is no distinct ‘head’ as in Nereis and the first body segment is called peristomium which bears the mouth aperture on its ven­tral surface. Overhanging the mouth on the dorsal surface is a small fleshy lobe, the prostomium, which is considered as a pro­jecting part of the first segment and not a segment by itself (Fig. 17.15). The last seg­ment bears the anus at its posterior end.

The apertures in the body are:

(a) Mouth:

A crescent-shaped aperture situated ventrally in the peristomium;

(b) Anus:

A round aperture situated at the posterior end of the last segment;

(c) Female genital aperture:

A single ap­erture situated on the mid-ventral line of the 14th segment;

(d) Male genital apertures:

Paired aper­tures situated on the ventro-lateral sides of the 18th segment. Each of these male genital apertures is associ­ated with one pair of genital papillae, situated above and below the aper­ture, i.e., on 17th and 19th segments;

(e) Spermathecal apertures:

Four pairs, situated on ventro-lateral sides of in­ter-segmental grooves between seg­ments 5/6, 6/7, 7/8, 8/9;

(f) Dorsal pores:

These apertures are present in the mid-dorsal line of the inter-segmental grooves of the seg­ments, between thirteenth to last but one;

(g) Nephridiopores:

Are numerous and open on the ventral surface of the body. They are present in all seg­ments excepting the first six and the last one.

3. Locomotor Organs of Earthworm:

Locomotor organs in case of earthworms are called setae. Each seta is an elongated more or less ‘S’-shaped rod composed of chitin, hardened and strengthened by the addition of sclerotised protein and is embed­ded in an epidermal pit, called setigerous sac or setal sac. The distal end of the seta is pointed but the proximal end is blunt. The middle of the seta is swollen and is called the nodulus.

They are about 0.24 mm in length and 0.03 mm in breadth. The number of setae in different segments varies considerably. Usually they are numerous (about 80-120) in each segment and are disposed in the form of a ring round each segment (Fig. 17.16A). This type of arrangement of setae in each segment is called perichaetine.

In Lumbricus, one pair of setae is found on each side of each segment and this type of arrangement of setae is called lumbricine. About two- third of the seta remains inside body wall and one-third of it projects out (Fig. 17.16B).

The inner part is attached to muscles respon­sible for moving the setae. Setae are present in all segments excepting the first, last and the clitellar segments. Setae are organs of locomotion on ground and act as attaching structures in burrows.

4. Body Wall of Earthworm:

The body wall is covered externally by a thin non-cellular cuticle composed of paral­lel layers of collagenous fibres and is perfo­rated by numerous pores through which open the epidermal glands. Histologically, it consists of two layers separated by an inter­vening layer. The cuticle is secreted by the supporting cells of the underneath epider­mis. Below the cuticle is the single-layered epidermis (Fig. 17.16B).

The cells that are found in the epidermis are:

(a) Gland cells or goblet cells:

These are mucus-secreting cells which keep the skin moist and slimy. Gland cells are of two types—mucous cells and albumen cells;

(b) Supporting cells:

These are tall and large cells outnumbering the other cells;

(c) Basal cells:

Cells remain packed between gland cells and supporting cells;

(d) Sensory cells:

Occur in groups and act as receptor organs.

All epidermal cells are of columnar type. Below the epidermis there is circular muscle layer forming a thin continuous sheet. Circu­lar muscle layer is followed by longitudinal muscle layer running in parallel bundles. The bundles are separated from each other by a thin septum of connective tissue. The innermost layer of the body wall is formed by coelomic epithelium which is a thin mem­brane of single layer of cells.

5. Coelom of Earthworm:

The coelom or body cavity is extensive. But it is not continuous and is divided into a number of compartments by transverse partitions running between the body wall and the alimentary canal. These partitions or septa are vertical in disposition and are formed by double layers of peritoneum and numerous interlacing bundles of muscle fi­bres.

The communications between the various coelomic compartments are provided by sphincter apertures situated just dorsal to nerve cord in each septum. In normal condi­tion the sphincter muscles remain contracted keeping each coelomic compartment separated from each other.

The coelom opens to the exterior by dorsal pores and nephri­diopores. The number of coelomic compart­ments corresponds to the number of external segments. However, in the first four seg­ments, the coelom is continuous as the septa are lacking there.

The coelomic compartments remain filled with coelomic fluid. The coelomic fluid is milk-white in colour and consists of plasma and four types of nucleated corpuscles (Fig. 17.17).

The corpuscles are:

(a) Phagocytes:

These cells are saucer- shaped granular, large and are most numerous;

(b) Circular nucleated cell:

These cells are non-granular;

(c) Mucocytes:

These are flat, circular and slender cells with fan-like process;

(d) Chloragogen cell:

These small but numerous cells have bulgings. When stained with iodine solution, they become yellow.

6. Locomotion of Earthworm:

The locomotion of earthworm is of interest since it gives an opportunity to study the results of co-ordinated movements in an ani­mal built on a metameric plan.

When an earthworm starts to crawl, the first few seg­ments become thinner and longer. This is caused by contraction of circular muscles and relaxation of longitudinal muscles of that re­gion; the opposing sets of muscles are antago­nised by an increase in the pressure of the coelomic fluid.

The thinning and elongation of the body gradually spread to more poste­rior segments. At this stage the setae of the anterior region are protruded to grip on the substratum. The longitudinal muscles of the anterior region then contract so that the body at that region becomes shorter and stouter and more hinder regions are pulled forward.

The septa also play an important role as they act as water-tight partitions which relay pressure changes from one segment to the next by bulging.

7. Digestive System of Earthworm:

The alimentary canal is a straight tube running from the anterior mouth to the pos­terior anus (Fig. 17.18). The mouth is a crescent-shaped aperture situated ventrally on the peristomium. Mouth opens into a short, thin-walled buccal cavity. The outer wall of the buccal cavity leads into the pharynx lying in segments 3rd and 4th. The pharynx is pear-shaped and its walls are thick and muscular.

The dorsal wall of the pharynx is lobulated and richly vascular. This lobulated part is called pharyngeal bulb. The pharynx is followed by oesophagus extending up to 8th segment. The part of the oesophagus lying in the 8th segment has become modi­fied to form an oval structure called gizzard.

The gizzard is thick-walled, muscular and its inner lining epithelium bears a distinct cuti­cle (17.19A). The food particles are ground against the cuticle by muscular efforts and become finer. The part of the alimentary canal lying between segments 9 to 14 is called stomach. The wall of the stomach is highly glandular and vascular. Both the ends of the stomach are provided with sphincter muscles.

The alimentary canal behind the stomach is wide, thin-walled and continues as intestine up to the anus in the last seg­ment. A pair of short and conical intestinal caeca is situated at the 26th segment. The dorsal wall of the intestine between 26th and 95th segment is infolded to form the typhlosole so that in cross section this part of the intestine appears to be ‘U’-shaped (Fig. 17.19B).

On the basis of the position of the typhlosole the intestine may be divided into pretyphlosolar region (segments 15th— 26th), typhlosolar region (segments 26th— 95th) and post-typhlosolar region (96th to last). The role of the typhlosole is to in­crease the surface of absorption. The outer wall of the intestine appears yellowish in colour because of the presence of chloragogen cells which line the intestine.

Histology of the gut wall:

The outer­most layer of the gut wall is made up of tall and narrow cells which are derived from peritoneal epithelium. This layer often remains covered by chloragogen cells, laden with yellow pigments. These yellow cells are be­lieved to be excretory in function. Some workers believe that these cells, in addition to their excretory function, take up the role of digestive glands.

The peritoneal epithelium is followed by longitudinal muscle fibres and circular muscle fibres. The internal epithelium is formed of glandular and ciliated cells.

Feeding and digestion:

During burrow­ing earthworm swallows soil rich in organic particles, seeds, decaying leaves, ova and larvae of small animals. At night they come out of their holes to feed on leaves and other vegetable matters. The leaves are seized by the pointed end of the mouth and held against the mouth by suction exerted by the pharynx.

Before the leaves are taken in, they are moistened by mucus secreted from the pharyngeal bulb. The food is then forced to the oesophagus by peristalsis. In the oesopha­gus, enzymes are added.

The food then reaches the gizzard whose cuticle renders the food particles finer by grinding. From the gizzard the food enters into the stomach for digestion and then to intestine for absorption. Insoluble remains of food together with soil that has been ingested are pushed out through anus.

8. Respiratory System of Earthworm:

Definite respiratory organs in earthworm are absent but gaseous exchange for respira­tion takes place mainly through the skin which is richly supplied with blood vessels and is kept moist by the secretion of epider­mal gland cells and by coelomic fluid escap­ing through the dorsal pores. Carbon diox­ide is also carried by blood to the skin from where it is eliminated.

9. Circulatory System of Earthworm:

The blood has the same composition as in Nereis, i.e., it is made up of plasma having haemoglobin in dissolved state and colour­less nucleated corpuscles suspended in plasma. The blood is red in colour.

The blood vascular system of earthworm (Pheretima sp.) is of closed type, i.e., the blood flow occurs through closed blood ves­sels, such as arteries, veins and capillaries and never directly comes to the tissue of the body.

In the body of the earthworm there is an elaborate circulatory system formed by closed tubes or blood vessels. The arrangement of the vessels is very complicated. The general plan is that longitudinally running vessels act as collecting or distributing vessels and these are connected with one another by transverse vessels in the individual seg­ments.

There are four pairs of such trans­verse vessels which are called ‘hearts’. As these are not like true hearts of the verte­brates it is better to call them pseudo-hearts. The arrangements of blood vessels in the first thirteen segments and in the rest of the body offer points of contrast. So for the sake of convenience they are described separately.

Blood vessels in the intestinal region or segments beyond thirteenth:

In this region of the body three large main vessels run longitudinally and parallel to each other (Fig. 17.20B).

(a) Dorsal vessel:

This large vessel runs in the mid-dorsal line of the body and just above the alimentary canal. The direction of movement of blood in this vessel is from behind forward.

The dorsal vessel is a col­lecting vessel and in each segment it re­ceives:

(i) A pair of dorsointestinal vessels in each side bringing blood from the intestine, and

(ii) A commissural vessel running along the posterior border of each septum and joined ventrally to the sub-neural vessel.

The vessel collects blood from skin and nephridia. The walls of the dorsal vessel are muscular and its lumen is provided with valves. In each segment there is a pair of valves di­rected forward and inward to prevent back- flow of blood.

(b) Ventral vessel:

This is a long vessel which runs in the mid-ventral line beneath the intestine. The direction of flow of blood is from anterior or posterior.

It serves as a distributing vessel and distributes blood through:

(i) A pair of ventrotegumentary branches one on each side of each septum. Each branch pierces the septum behind it and runs upward to supply blood to inner body wall and integumentary nephridium. Each ventrotegumentary branch gives a septo-nephridial branch which runs on the anterior face of the septum supplying blood to septal-nephridia,

(ii) A median ventro- intestinal is given off dorsally from the ven­tral vessel. It supplies blood to the floor of the intestine. Valves in the lumen of ventral vessel are absent.

(c) Subneural vessel:

It is a long and slender vessel running along the mid-ventral line beneath the nerve cord. The flow of blood is from front to backward. It is a collecting vessel. It collects blood in each segment by a pair of ventral branches, which collect blood from ventral skin.

The sub- neural vessel is linked to dorsal vessel by a pair of commissural vessels in each segment. Extending from the posterior end to the 14th segment, it bifurcates into two to form the lateral oesophageal vessels.

Blood plexus of the intestine:

The intestinal walls are traversed by a close network of capillaries. The capillary net­works are constituted by an internal network between circular muscle layer and internal epithelial lining of intestine and an external network on the outer surface of the intestine.

Commissural, integumentary and ne­phridial vessels:

The commissural vessel gives off from its ventro-lateral side a septo-intestinal branch to the internal plexus. The branches of the in­tegumentary vessels from body wall and ne­phridial vessels open into the internal plexus.

Blood vascular system in the first thir­teen segments:

The first thirteen segments have the fol­lowing vessels (Fig. 17.20A).

(a) Dorsal vessel:

It becomes the distrib­uting vessel at this region and extends up to the cerebral ganglion. It gives off branches which supply the anterior regions of the body. It sends back blood to the ventral vessel through the ‘hearts’.

(b) Supra-oesophageal vessel:

It is the shortest vessel extending between segments 9th and 13th. It is a collecting vessel and collects blood from gizzard and stomach.

(c) Lateral oesophageal vessel:

These vessels have been formed by the bifurcation of the sub-neutral vessel at the 14th segment. These are collecting vessels and continue anteriorly along the lateral sides of the oesophagus. Through a pair of branches in each segment they collect blood from lateral regions of gut, body wall, septum and seminal vesicle.

(d) Ventral vessel:

It extends anteriorly up to the 2nd segment and is a supplying vessel. It supplies blood to ventral body wall, septal nephridia and reproductive or­gans. It gives a pair of ventrotegumentary vessels in each segment.

Hearts and anterior loops:

The dorsal and ventral vessels are con­nected to each other in segments 7th, 9th, 1:2th and 13th by means of paired, muscular, neurogenic and pulsatile structures called ‘hearts’ or called pseudo-hearts because these do not function like, the way a vertebrate heart works (Fig. 17.21).

The posterior two pairs situated in 12th and 13th segments are called latero-oesophageal hearts because they communicate dorsally both with dorsal and supra-oesophageal vessels while the an­terior pairs situated in the 7th and 9th seg­ments are called lateral hearts because they communicate directly the ventral vessel to the dorsal vessel.

Besides these, there are two pairs of non-­pulsatile and non-muscular loops situated in 10th and 11th segments. These loops are without valves and connect the supra-oesophageal to the lateral oesophageals.

Ring vessels:

The wall of the stomach between seg­ments 10th and 13th bears dozens of circular vessels in each segment. Blood from lateral oesophageals to the supra-oesophageal is carried by these vessels.

Mechanism of blood circulation:

The movement of blood through the dif­ferent vessels is carried on by peristaltic contractions (Fig. 17.22). There is a pair of ring valves in front of each septum directing blood forward or backward as the case may be, which prevent back flow. Blood is carried to the skin by the ventrotegumentary branches from the ventral vessel for aeration.

Oxygenated blood is collected by the paired branches of sub-neural vessel (collecting vessel) and by the capillaries of the commissural, which in their turns open to the dorsal vessel.

Moreover, the dorsal vessel also receives the dorso-intestinals from the intestine in the in­testinal region where it acts as a collecting vessel. In the first thirteen segments the dorsal vessel becomes a distributing vessel and pours blood in the ventral vessel through hearts. The ventral vessel does not distribute blood.

Blood-forming glands:

Behind the pharyngeal mass in segment 4th and behind it in 5th and 6th segments, there exists an aggregate of spherical folli­cles, called blood glands. Each follicle con­sists of a capsule of syncytial layer of multi­nucleated protoplasm having in its concav­ity a loose mass of cells. The blood glands are considered as manufacturers of blood cor­puscles and haemoglobin.

Excretory System of Earthworm:

Excretory organs are the nephridia. Nephridia occur in all segments excepting the first three and the last segments. The ne­phridia are small and coiled tubular struc­tures and occur in huge numbers.

In the body of earthworm three kinds of (Fig. 17.23) nephridia occur:

(a) Septal ne­phridia,

(b) Integumentary nephridia and

(c) Pharyngeal nephridia.

Structurally, these nephridia show basic similarities; its classification is based on its position in the body.

The types of nephridia that occur in earthworm are described below:

(a) Septal nephridia:

Septal nephridia remain attached to the two faces of the septum. They occur from 15th segment back­ward. That means in the first fourteen seg­ments they are absent. Each septum bears 40-50 nephridia in average in its anterior and a similar number on its posterior face. Thus in each segment there are about 80-100 nephridia.

Structure of septal nephridium:

A typi­cal septal nephridium (Fig. 17.24A) consists of a main body formed by a straight lobe and a long narrow, spirally twisted loop, a fun­nel-like nephrostome connected to the main body by a short neck and a terminal ne­phridial duct. All the nephridial structures remain restricted to the same segment.

The nephrostome or ciliated funnel is a rounded structure. The mouth of the funnel which communicates with the coelom is pro­vided with a large upper lip and a small lower lip. The lips are ciliated and are provided with several rows of ciliated marginal cells.

A narrow ciliated tube runs from the funnel into the body of the nephridium, called neck and takes several turns inside it.

The main body of the nephridium is made up of a main lobe and a spirally twisted loop. The loop is twice as long as the main lobe and consists of a proximal limb and a distal limb twisted round each other.

The straight lobe is continued as the distal limb of the twisted loop and the proximal limb receives the ciliated tubule from the nephrostome and also it gives off the termi­nal duct which opens at the nephridiopore. The straight lobe bears four parallel tubules, the proximal and distal ones bear three tu­bules each and in the apical part there are two tubules.

Terminal ducts of the septal nephridium open into a septal excretory canal which runs parallel and internal to commissural vessels.

There are a pair of septal excretory canals one on each side of the septum. The two septal excretory canals open into a pair of supra-intestinal excretory ducts which run on the mid-dorsal line side by side from 15th segment to the posterior end. The supra- intestinal excretory ducts open into the lu­men of the intestine by single and small ducts at the level of each inter-segmental septum.

(b) Integumentary nephridia:

They are smaller in size than the septal nephridia. These ‘V’-shaped structures occur on the inner surface of the integument in all seg­ments excepting the first two.

They number 200-250 in each segment but in the 14th, 15th and 16th segments the number of nephridia is much more. Structurally they resemble septal nephridia but lack the nephrostome. They open independently to the outside by nephridiopores on the outer surface of the body wall.

(c) Pharyngeal nephridia:

They are as large as the septal nephridia and occur in the form of three pairs of bunches or tufts in the 4th, 5th and 6th segments and on either side of pharynx and oesophagus.

Nephrostomes are also absent in the pharyngeal nephridia. In each bunch the terminal ducts of the nephridia join together to form a slender duct. The slender ducts again unite in each segment and form a thick-walled duct which opens into the alimentary tube.

Thus there are three pairs of ducts, one pair each in the 4th, 5th and 6th segments. Some workers maintain that the pharyngeal nephridia have digestive function or, in other words, they aid in digestion and hence they are some­times referred to as ‘peptic nephridia’.

The sepal and pharyngeal nephridia open into the alimentary canal and are called enteronephric while the integumentary ne­phridia open to the outside directly and are called exonephric. The enteronephric sys­tem helps in the conservation of water in the

Body because water present in the excretory product is again reabsorbed in the intestine.

Some of the nitrogenous excretory substances like guanin are extracted from the blood stream by chloragogen cells. These cells collect and strore excretory prod­ucts and on becoming heavily laden with excretory materials, they pinch off into the coelomic fluid from where they are elimi­nated through dorsal pores or by nephridiopores.

10. Nervous System of Earthworm:

The nervous system (Fig. 17.25) comprises of a central nervous system from which peripheral nerves are given to the different parts of the body and the receptor organs.

Central nervous system:

It consists of a pair of closely packed suprapharyngeal ganglia or cerebral gan­glia forming the brain and situated in the third segment of the body above the phar­ynx. From the brain nerves are given off to the protomium and walls of the buccal cham­ber. Two loops called circum pharyngeal connectives arise, one from each ganglion of the brain.

They encircle the pharynx and meet with a pair of sub-pharyngeal ganglia lying below the pharynx and in the anterior part of the fourth segment. The connectives give off nerves to the first segment of the body and to the buccal chamber, and the sub-pharyngeal ganglia give branches to sup­ply the second, third and fourth segments.

A ventral nerve cord extends from the sub-pharyngeal ganglia to the last segment of the body and runs along the mid-ventral line. The nerve cord, though appears single, is in reality made up of two cords. Behind the fourth segment the cord presents a swell­ing in each segment.

Each swelling is formed by fusion of paired ganglia. From each of these ganglia, three pairs of peripheral nerves are given to the body wall and vis­cera. These nerves contain both afferent and efferent fibres.

Receptor organs:

The structures which receive stimuli and convey the same to the central nervous system are called receptors.

The receptors found in the body of earthworm are described below:

(a) Epidermal receptors:

These receptors are placed in the epidermis as groups of long slender receptor cells (Fig. 17.26A) which are tactile in function.

(b) Photoreceptors:

Each photoreceptor (Fig. 17.26B) is a single cell with a distinct nucleus and an optic organella. These receptor cells are housed in the epidermis. The cells are numerous at the anterior end of the body. The number gradually decreases posteriorly. The last segment may even contain one such cell. The receptor cells are absent on the ventral surface.

(c) Buccal receptors:

Groups of cells are placed in the buccal cavity in large numbers. These receptors are concerned with chemical stimuli and serve in smelling and tasting food particles (Fig. 17.26).

11. Reproductive System of Earthworm:

Earthworm is monoecious or hermaphrodites, i.e., male and female reproductive structures develop in the same individual.

I. Male reproductive system:

Male reproductive system consists of the following organs, such as:

(i) Testes,

(ii) Tes­tis sacs,

(iii) Seminal vesicles,

(iv) Vas defer­ens, and

(v) Prostate gland.

(i) Testes:

There are two pairs of testes situ­ated in the 10th and 11th segments. They are found one on either side of the nerve cord. The testes are white digited structures and each testis is made up of testicular lobules or follicles. These follicles contain spermatogo­nia. Sperm mother cells or spermatogonia are discharged in the cavities of the testis sacs.

(ii) Testis sacs:

Two pairs of testis sacs are located beneath the oesophagus in 10th and 11th segments (Fig. 17.27A). These are large, thin walled, sac-like structures and light yellow in colour.

(iii) Seminal vesicles:

There are two pairs of seminal vesicles placed laterally in the 11th and 12th segments. The testis sacs of the 11th segment are comparatively larger and com­municate to the vesicles of their own sides. The testis sacs situated on the right side of the 10th segment communicates to the vesi­cle of the right side in the 11th segment and so on.

In each testis sac, the reproductive organ, testis remains attached on the inner surface of the anterior end of the sac. From the testis sac the spermatogonia go to the seminal vesicles for maturation and after maturation they again come back to the sacs.

(iv) Vasa deferentia:

Behind each testis and enclosed in the testis sac there is a ciliated spermiducal funnel. The funnel leads into a slender duct, called spermiduct or vas def­erens. There are four vasa deferentia.

The two vasa deferentia of each side continue posteriorly from the 12th to 18th segment and enter into prostate gland. The terminal portion of the duct is enlarged to form a muscluar atrium and sometimes there is an eversible penis.

(v) Prostate glands:

There is a pair of large, flat and creamy white glands, called prostate or spermiducal glands associated with male ducts and are extended usually from 17th to 21st segments. Each gland is an irregular shaped structure and can be differentiated into a large glandular part and a small non- glandular part.

Each gland gives out a duct. This duct along with the two vasa deferentia of its own side becomes enclosed in a com­mon muscular-sheath and opens through the male genital aperture situated in the 18th segment. The prostate glands secret, a fluid in which the sperm cells are nourished.

Accessory glands:

There are two pairs of rounded accessory glands situated on 17th and 19th segments in association with pros­tate glands. Each segment bears a pair of glands located on either side of the nerve cord and remains attached to the ventral side of the body. They open to the exterior by a number ducts through the two pairs of geni­tal papillae. These glands secrete a kind of secretion which helps in copulation.

II. Female reproductive system:

The female reproductive system consists of the following organs, such as:

(i) Ovaries,

(ii) Oviducal funnels, and

(iii) Spermathecae or Seminal receptacles.

(i) Ovaries:

There is a pair of Ovaries attached to the hinder face of the septum lying between the 12th and 13th segments (Fig. 17.27B). The ovaries are small, white digited structures and contain ova being at different stages of development. The ma­tured ova remain towards the closed end and the young ova near the proximal end. The ovaries are larger than the testes.

(ii) Oviducal funnels:

Beneath each ovary lies an oviducal or ovarian funnel. The ovi­ducal funnels are saucer-shaped with cili­ated and fimbricated margins. The funnels lead behind into conical ducts, and two oviducts unite to form a common oviduct which opens to the outside by the genital aperture situated ventrally on the 14th seg­ment.

(iii) Spermathecae or Seminal receptacles:

Though the spermathecae or seminal recep­tacles form a part of female reproductive system but these structures are not con­nected with the oviducts and remain completely separated. There are four pairs of spermathecae in the 6th to 9th segments of the body. The spermathecae are flask-shaped having each pair in each segment.

Each spermatheca is small and consists of a pear- shaped ampulla and a narrow duct with a small, blind diverticulum. The ducts open to the outside through spermathecal pores situated in the segments between 5/6, 6/7, 7/8 and 8/9 segments. The sperms are stored in the spermathecae received from the other earthworm during copulation.

Copulation:

Though hermaphrodite, earthworm prac­tices cross-fertilization. Mating occurs at the onset of rainy season. The clitellum contains glandular cells, the secretion of which pro­vides mucus for copulation.

During mating, two individuals come together and oppose each other in a head to tail position. The male genital apertures of one rest on the spermathecal pores of the other. After mu­tual exchange of sperms the individuals sepa­rate and sperms remain stored in the spermathecae.

Cocoon formation and development:

The cocoon formation in Pheretima has not well studied but is presumed that similar process takes place like Lumbricus or other terrestrial species. Clitellar glands secrete some substances which are cemented to­gether to form a membranous girdle round the clitellar region. The earthworm now tries to pull itself out of the girdle.

When the girdle slips over the oviducal aperture, ova are discharged into it and as it passes over the spermathecal apertures, sperms are also discharged. The animal pulls its body out of this girdle.

The two open ends of the girdle due to elasticity become closed and a cocoon is formed. Fertilization and development occur inside the cocoon. Generally, one indi­vidual comes out of one cocoon. Develop­ment is direct and the developing embryo gets nourishment from stored albumen.