An Example of Phylum Annelida: Leech

In this article we will discuss about Leech:- 1. Habit and Habitat of Leech 2. External Structures of Leech 3. Body Wall 4. Coelom 5. Locomotion 6. Digestive System 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System 11. Receptor Organs 12. Reproductive System.

Contents:

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

1. Habit and Habitat of Leech:

Several kinds of leeches are found in our country. The type which is discussed here is commonly called cattle leech and its scien­tific name is Hirudinaria granulosa. It is a member of the class Hirudinea. These leeches are seen in swamps, ponds and slow-flowing streams. It is sanguinivorous (lives on blood meal) and ectoparasitic in nature.

2. External Structures of Leech:

Body is elongated and vermiform (Fig. 17.28). Body appears cylindrical in contracted state and dorso-ventrally flattened in ex­tended condition. A full-grown leech meas­ures 35-40 cm in length. Leeches are brightly coloured. The dorsal surface is olive-green and the ventral surface is orange-yellow or yellow in colour.

The dorsal surface also bears stripes or orange or yellow. On the mid-dorsal surface there is a long stripe extending from the first pair of eyes to the anus. This stripe is either uniform or inter­rupted. On either side of this median stripe there are four discontinuous wavy stripes extending from tip to tip of the body.

The stripes, running along the outermost sides of the body, bear supra-marginal spots on the 2nd and 5th annuli of each segment. These spots offer keys to taxonomy. The first and second stripes on each side of the middle stripe are irregular and inconspicuous.

These stripes have been termed as inner parame­dian, outer paramedian, intermediate, and supra-marginal. The first annulus of each segment bears a ring of segmental receptor organs or segmental papillae in the form of small elevations.

Numbers of segments in the body are fixed and they are thirty-three. These true segments have been divided externally into annuli by superficial furrows. Each segment bears three to five such external furrows.

The body may be divided into following regions:

(a) Cephalic or Head region:

It is consti­tuted by five anterior segments (Fig. 17.28B). The two anterior-most segments (1st and 2nd) are not divided into annuli. The third seg­ment is bi-annulate while both of the fourth and fifth segments are tri-annulate. The ce­phalic region includes the anterior sucker or oral sucker and the eyes.

There are five pairs of eyes; the first and second pairs are situated on the 1st and 2nd segments. The third, fourth and fifth pairs of eyes are situated on the 1st annulus of 3rd, 4th and 5th segments respectively. The anterior sucker is situated on the ventral surface of the cephalic region. It is cup-shaped, oval in outline and is formed by the fusion of anterior five segments on the ventral surface.

At the bottom of the cup- shaped sucker lies the mouth which is nar­row and tri-radiate. At the tip of the cephalic region and above the first segment is situ­ated the prostomium which serves as a sort of upper lip to the mouth and bears glands and sense organs for touch and taste.

(b) Pre-clitellar region:

It is constituted by segments 6th, 7th and 8th. The 6th seg­ment has three annuli while in the other two there are five annuli. All these segments bear a pair of nephridiopores on the ventral surface.

(c) Clitellar region:

The 9th, 10th and 11th segments form the clitellar region. The 10th segment bears in its mid-ventral line the male genital aperture while the female genital ap­erture is situated on the mid-ventral line of the 11th segment. Each segment has a pair of nephridiopores on the ventral surface. Perma­nent clitellum is lacking in leeches. Clitellum becomes prominent in breeding season.

(d) Middle Region:

The eleven segments starting from 12th to 22nd are provided with five annuli and nephridiopores.

(e) Caudal Region:

It is made up of segments from 23rd to 26th (Fig. 17.8C). The 23rd segment is with three annuli and others are with two. On the dorsal surface of the 26th segment a median aperture, anus, is present in the furrow between two annuli. Nephridiopores are absent.

(f) Posterior sucker region:

Constituted by seven segments from 27th to 33rd. None of the segments is annulated and all the segments are indicated by seven circles. Seg­mental receptors are very prominent on the suckers. The posterior sucker is much larger and more powerful than the anterior one.

3. Body Wall of Leech:

The outer covering of the body wall is cuticle which is a non-cellular, thin and transparent membrane secreted by the epi­dermis (Fig. 17.29A). It is perforated by many pores. The epidermis is made up of a single layer of cells. The cells are hammer-shaped. The ‘head’ or flat surfaces of these cells are directed towards the cuticle while the ‘han­dle’ is directed inwards.

The spaces between the inner ends of the epidermal cells are occupied by fibrous connective tissues con­taining blood and capillaries. Embedded in the connective tissue are unicellular epider­mal glands. Groups of cells become modi­fied to form receptor cells.

Four types of glands are recognised:

(a) Slime glands:

Distributed all over the surface and secrete slime. They may be “pear-shaped” or “tubular” in appearance.

(b) Sucker glands:

Situated in masses on anterior and posterior suckers.

(c) Prostomial glands:

These are placed in groups on the prostomium, the secretions form plugs during cocoon formation.

(d) Clitellar glands:

These are present in the clitellar region. The secretory activity of glands increases during breeding season.

The glands open to the outside through the pores on the cuticle. Epidermis is fol­lowed by a thick dermis which is an assem­blage of fibrous connective tissue contain­ing short muscle fibres, haemocoelomic si­nuses and capillary networks. The circular and longitudinal muscle layers follow as usual. Besides these, there occur oblique, dorso-ventral, radial and vertical muscles.

These muscles are arranged either singly or in bundles (Fig. 17.29B). A peculiar type of connective tissue, called botryoidal tissue, occurs between enteric canal and longitudi­nal layer of muscles. The tissue consists of large and perforated cells arranged end to end so as to constitute minute intracellular canals filled with haemocoelomic fluid. The cells are heavily pigmented and appear black in colour.

The detailed histological structure of the body wall is best seen in transverse sections passing through the body (Fig. 17.30). A transverse section from the middle of the body shows above-mentioned structures of the body wall. Besides, in such sections it is seen that the major part of the body space is occupied by the crop and its lateral diver­ticula, the caeca.

The crop and the diverticula are lined internally by endoderm which is projected towards the lumen of the alimen­tary canal. The lateral sides are occupied by nephridia.

Below the nephridia run the lat­eral haemocoelomic channels. Above the crop lies the dorsal haemocoelomic channel while below the crop lies the ventral haemocoelomic channel. The ventral nerve cord passes through the ventral haemocoelomic channel.

4. Coelom of Leech:

An extensive and spacious perivisceral coelom is absent in leeches. The coelom is greatly reduced by the development of con­nective tissue between the alimentary tube and the body wall. The coelom becomes restricted to four longitudinal channels, two of which are lateral, one dorsal and the remaining one is ventral in position.

These channels contain coelomic fluid which is red in colour with haemoglobin and corpuscles. So the coelomic fluid is better called haemocoelomic fluid and the channels, as the haemocoel. True coelom is represented by spaces enclosing the gonads and reproductive ducts.

5. Locomotion of Leech:

The leeches can move about from place to place by (a) swimming and (b) crawling (Fig. 17.31).

(a) Swimming:

Leeches swim about el­egantly in water by snake-like lateral undulations of the body. Bi-lateral waves occur by the contraction of dorso-ventral muscles and relaxation of longitudinal muscles.

(b) Crawling:

Leeches crawl on substra­tum under water as well as on land by looping. The suckers play their part alternately as adhesive organs. Actual locomotion is affected by al­ternating changes in the length of the body.

During crawling, the posterior sucker first gets attached to the substratum. This pro­duces a wave of excitation and contraction of the circular muscles and simultaneous re­laxation of longitudinal muscles. As a result, great extension of the body length occurs. The animal now fixes the forwarding ante­rior sucker on the substratum.

Now contrac­tion of longitudinal muscles and relaxation of circular muscle occur and the posterior sucker is released from the substratum. The contracting longitudinal muscles drag the posterior sucker forward and the body as­sumes the shape of a loop or inverted ‘U’. The posterior sucker is now fixed in the new position and the animal moves forward by the repetition of the process.

6. Digestive System of Leech:

It includes a long alimentary canal and digestive glands in the form of salivary glands (Fig. 17.32). The alimentary canal starts from mouth. Mouth is a tri-radiate aperture situ­ated in the middle of the anterior sucker and leads to a very small buccal chamber. The walls of the buccal chamber bear three jaws one of which is dorso-median in position and the other two are ventro lateral.

The jaws are laterally compressed muscular cushions having chitinous coverings. The chitinous covering is produced into rows of teeth. The jaws can be operated by muscles for back­ward and forward movements. Both sides of a jaw are provided with papillae which bear the openings of the salivary glands.

The buccal chamber leads to the pharynx which extends from the 5th to the 8th segments. The entire space between pharyngeal wall and body wall is packed with numerous salivary glands. The glands are unicellular and open in the papillae by the side of the jaws through ducts.

The secretion of the glands prevents the coagulation of blood of the prey during feeding. Radial muscles run between the walls of the pharynx and body wall which operate to dilate the pharynx and accelerate suction. The pharynx opens into the largest part of the alimentary canal—the crop through a small oesophagus.

The crop extends from 9th to 18th segments. It is represented by ten chambers arranged metamerically one in each segment. Each cham­ber opens behind into the succeeding cham­ber by a round aperture controlled by sphinc­ter. A pair of blind pockets or caeca project out laterally from each chamber.

The caeca increase in size from before backward. The first pair is the smallest and the last pair is the largest. The last pair of caeca extends as far as 22nd segment. The crop opens into the stomach which is small, tapering and res­tricted to the 19th segment. The walls of the stomach are produced internally into trans­verse folds.

The stomach continues as intes­tine occupying 20th to 22nd segments. It is a narrow and straight tube and its internal wall bears longitudinal and transverse folds to increase the surface of absorption.

The intestine narrows down at the posterior end and enters into the rectum. It extends from 22nd to 26th segments. It is thin-walled and it opens to the outside through anus which is placed on the dorsal surface of the 26th segment.

Feeding and Digestion:

Leech is an ectoparasite animal and lives on the blood of vertebrates. A crop full of blood may weight about 300gm and is digested and absorbed in several months.

A leech gets attached to the body of its prey by the posterior sucker. The cup-shaped ante­rior sucker is then suitably placed to the soft site on the host skin. The jaws are protruded and their serrated margins then play on the skin of the prey by the action of muscles.

Ultimately a tri-radiate wound is made and blood comes out. This blood is sucked into the pharynx by alternate contraction and expansion of pharyngeal muscles.

The secretion of the salivary glands contains an anti-coagulating agent, hirudin, which pre­vents coagulation of blood and thus a con­tinuous flow is maintained. Blood passes into the crop and caeca and is stored there. The stored blood inside the crop becomes transformed into a jelly-like mass by the process of haemolysis.

A drop or two of the blood enter into the stomach from the 10th crop and the flow is controlled by sphincter. The blood is di­gested here and changed into a green mass. Proteolytic enzymes secreted by the stom­ach help in digestion. The digested food is absorbed in the intestine and the residue is thrown out as black mass through the anus.

7. Respiratory System of Leech:

As in earthworm, the skin serves as res­piratory organ in leeches. The skin is richly supplied with haemocoelomic vessels and being permeable, the carbon dioxide of the haemocoelomic fluid in the capillaries is exchanged for oxygen dissolved in water. In some leeches as Branchellion, lateral outgrowths of the body wall serve as gills for respiration.

8. Circulatory System of Leech:

A closed blood-vascular system like that of earthworm is absent in leeches. The blood vessels are represented by coelomic chan­nels, filled with blood-like fluid. These chan­nels form the haemocoelomic system. The coelomic fluid contains haemoglobin. Col­ourless corpuscles are circulated through these channels.

The haemocoelomic system consists of:

(1) Four longitudinal channels (Fig. 17.33), two of which run laterally, one on each side, one runs along the mid-dorsal line and the other along the mid-ventral line;

(2) Elabo­rate branches arise from these channels and

(3) A large number of spaces, some of which are coelomic, while the others are of un­known nature. The four channels are in di­rect communication to each other at the posterior part of the body.

Different Longitudinal Channels:

(i) Dorsal channel:

It runs along the mid- dorsal line beneath the body wall and re­mains firmly attached to the gut. The girth of the channel is more or less uniform. In each segment the dorsal channel gives two pairs of dorso-lateral branches.

The dorsolateral branches arise from the ventral surface of the dorsal channel and run to the dorsal and dorso-lateral sides of body where they form capillary plexus. The dor­sal channel gives a large number of small dorso-intestinal branches to the alimentary canal.

At the 6th segment the dorsal channel breaks up and forms a network of capillaries in the anterior five segments. At the 22nd segment the dorsal channel becomes bifur­cated. These branches pass downward round the rectum to enter into the posterior dilata­tion of the ventral channel. Thus a direct communication between the dorsal and ven­tral channel is established.

The dorsal channel and its branches are devoid of muscular walls and valves. The flow of fluid is from posterior to anterior end.

(ii) Ventral channel:

It runs beneath the alimentary canal and encloses the ventral nerve cord. Anteriorly, it encloses the cerebral ganglia, peri-pharyngeal nerve ring and the sub-pharyngeal ganglia. Posteriorly, it en­closes the terminal ganglia. The ventral chan­nel extends from anterior to posterior and is larger in diameter than the dorsal channel.

In each segment, the ventral channel gives a pair of branches from the level of each ganglion. Each branch bifurcates into two forming a ventral branch which forms a capillary network on the ventrolateral wall and in antero-dorsal branch which runs vertically upward and forms dorso-lateral capillary network on the skin.

In 6th to 22nd segments, the ventral chan­nel gives in each segment a pair of nephridial branches. The nephridial branches arise from ventral channel behind the nerve ganglion in each segment and run outwards. They form saccules, called peri-nephrostomial ampul­lae, around each testis sac and give off minute branches to nephridium and ultimately break up to form capillaries on the lateral body wall.

The ventral channel and its branches are thin-walled and without valves.

The direction of movement of haemo­coelomic fluid is from the anterior to the posterior end.

(iii) Lateral channel:

The two lateral channels which have muscular walls run symmetrically one on each side of the ali­mentary tube. The diameter of the channels is wider at the posterior third of the body.

Each lateral channel gives off in each segment a single latero-ventral branch which soon bifurcates to form anterior and poste­rior branches. These two branches join with their counterparts from the other lateral chan­nel in the mid-ventral line and beneath the ventral channel forming ventral commis­sure of the laterals.

There are eighteen such commissures in the segments between 6th and 23rd. The commissures of successive segments are in communication with each other through three inter-segmental and longitudinal commissural branches. Fig. 17.34 shows the disposition of lateral channels and their tributaries in a segment of leech.

There is no valve around the place of origin of the latero-ventral but valves are present in the place of origin of its branches.

The lateral channel receives a latero-lateral and latero-dorsal branch in each segment. The latero-lateral branches come from lateral body wall and from the nephridium of the same side and segment.

The latero-dorsal is a large branch formed by the union of branches coming from dorsal and lateral body wall, the gut and nephridium of the same side and segment. Each latero-dorsal meets it’s fellow from the opposite side above the dorsal chan­nel to form dorsal commissures of the lateral channels. There are seventeen such loops between 6th and 23rd segments.

In each segment lateral commissures join the latero-dorsals and latero-laterals.

Both latero-dorsals and latero-laterals are collecting vessels and pour their contents into lateral channel of the same side. The place of entrance of latero-dorsal and latero- lateral into the lateral channel is guarded by valves. The movement of fluid in the lateral channels is from behind forward.

Anteriorly both the lateral channels break up in the 5th segment into capillaries but posteriorly they open into the dilation of the ventral channel where a direct communica­tion is formed between all the four channels (Fig. 17.35). Fig. 17.36 shows the disposition of haemocoelomic channels in transverse sections.

Mechanism of Blood Circulation:

The dorsal and ventral channels are distributary in function and the two lateral channels are both distributary and collecting in nature.

The dorsal channel supplies dorsal and dorso-lateral parts and the entire alimentary canal.

From dorsal and dorso-lateral body wall haemocoelomic fluid goes direct to the lat­eral channel through latero-dorsals. From alimentary canal the fluid runs through two latero-intestinals via dorsal commissures of the laterals.

The ventral channel supplies the ventral, ventro-lateral body walls and nephridia. The fluid from these parts returns to the lateral channels through latero-lateral and latero-dorsal branches.

Each lateral channel distributes to the nephridia, genital organs, gut and ventral body wall through latero-ventrals and receives back from these regions through latero-laterals and latero-dorsals.

9. Excretory System of Leech:

Excretory system consists of nephridia. There are seventeen pairs of nephridia one pair each in 6th and 22nd segments. Thus the first five segments and the posterior four segments are devoid of nephridia. Of these seventeen pairs, the anterior six pairs lie in the pre-testicular segments and are called pre-testicular nephridia and the remaining eleven pairs in the testicular segments, are called testicular nephridia.

Structure of a testicular nephridium:

A typical testicular nephridium (Fig. 17.37) consists of:

(a) Main lobe,

(b) Apical lobe,

(c) Initial lobe,

(d) Ciliated organ,

(e) Inner lobe, and

(f) Vesicle duct and vesicle.

(a) Main lobe:

It is a horse-shoe-shaped structure and ventro-lateral in position. The main lobe consists of two unequal limbs, anterior and posterior. The anterior limb is larger in size than the posterior and the two together at their junction form the bend of the horse-shoe.

(b) Apical lobe:

The posterior limb of the main lobe passes forward to form the stout apical lobe. It lies in an antero-posterior position beneath the gut. Its anterior end is bent upon itself.

(c) Initial lobe:

It runs as an extremely long and slender lobe twined round the apical lobe. At its anterior extremity it runs as a slender thread of cells towards the testis sac and ends abruptly by the side of the perinephrostomial ampulla. While at its posterior extremity it joins the main lobe near the point of emergence of the vesicle duct.

(d) Ciliated organ:

The ciliated organ lies in the peri-nephrostomial ampulla. It is a compound structure made up of a central reservoir and ciliated funnels. The reservoir is spongy in nature. The ciliated funnels are present in a large number on the wall of the reservoir.

Each funnel is like an ear-lobe having a broad distal end and a proximal neck which fits into pores situated on the wall of the reservoir. The funnel is made of five to six cells arranged in two tiers and is densely ciliated. The ciliated organ has no excretory role in adult leeches and is associ­ated with haemocoelomic system.

(e) Inner lobe:

It lies in the inner concav­ity of the main lobbe and runs forward along the outer border of the apical lobe about half its length.

(f) Vesicle duct and vesicle:

The inner end of the anterior limb of the main lobe gives rise to a vesicle duct. The vesicle duct opens into the terminal vesicle. The vesicle is a large oval sac lying behind the nephridium and fixed to the ventro-lateral body wall. The vesicle leads into a narrow canal which perforates the body wall and opens to the outside by nephridiopore.

Histology of nephridium:

The nephridium bears internally a central canal and in each lobe there are cells sur­rounding the canal.

Main lobe:

Cells are large and polyhe­dral in shape. Each cell is provided with a narrow lumen which branches repeatedly and forms systems of fine capillaries which anastomose with those of neighbouring cells and open into the central canal.

Apical lobe:

Cells are large and radially arranged round the central canal. The cells are pierced by intracellular canals. These canals join with one another to form a net­work which opens at places into the central canal.

Initial lobe:

The initial lobe is very thin and is made up of a single row of elongated cells placed end to end. The cells are perfo­rated by a continuous intracellular canal which gives off several diverticula in each cell. The canal ends blindly at the testis sac while its other end opens in the main lobe close to the place of origin of the vesicle duct.

Ciliated organ:

The central reservoir is a spongy mass having an outer wall formed by a single layer of perforated cells enclosing a central mass of connective tissue cells and corpuscles. The funnels that fit in the pores of the outer wall are composed of five to six cells arranged in two tiers. The outer margin and the inner surface of each funnel are densely ciliated.

Inner lobe:

The cells are elongated and tubular. The pores inside the cells are exten­sive and as a result a thin peripheral cyto­plasm with nucleus is encountered. Five or six of these cells line the central canal.

Vesicle duct and vesicle:

The vesicle is thin-walled and is lined internally with cili­ated epithelium. The vesicle duct and the outgoing duct from the vesicle are both non- ciliated. A sphincter muscle is present round the opening of the outgoing duct from the vesicle.

Route of central canal inside the ne­phridium:

The central canal follows a long and zigzag course throughout the body of the nephrid­ium. It makes one complete and another incomplete round of run inside the body of the nephridium. The canal begins as a large intracellular lumen in a single cell at the anterior end of the apical lobe. Then it comes down the apical lobe to enter the inner lobe from where it goes of the poste­rior border of the anterior limb of the main lobe.

From the main lobe, it passes along the outer border of the apical lobe and on being embedded in it completes the first ’round’. The canal then loops backward and traverses the apical lobe. On reaching the posterior limit of the apical lobe it passes onto the posterior limbs of the main lobe.

After traversing the main lobe and forming many loops in it the canal emerges from the posterior lip of the anterior limb and continues into the vesicle as vesicle duct. The second part of the canal is incomplete by one-fourth ’round’.

Pre-testicular nephridium:

Structurally these nephridia resemble the testicular nephridia except that they lack ciliated organs. This is because the testis sacs in those segments are not formed. The initial lobes end loosely within the connec­tive tissue on each side of the ventral nerve cord.

Role of nephridium and ciliated organ:

In the body of leech the haemocoelomic fluid, i.e., blood and coelomic fluid, are not present as separate entities as they are in earthworm. The ciliated organ though con­sidered along with excretory structures is totally separated from nephridium in an adult leech.

The ciliated organ bathes in haemocoelomic fluid and has no excretory role. It is subservient to haemocoelomic sys­tem and manufactures haemocoelomic cor­puscles. While the body of the nephridium minus the ciliated organ is richly supplied with lateral channels, it is excretory and osmoregulatory in functions.

Many workers believe that the Botryoidal tissues are excretory in nature.

10. Nervous System of Leech:

The nervous system of Hirudinaria shows basic similarity to annelidian type as seen in earthworm (Fig. 17.38). But fusion of several anterior and posterior ganglia due to abbreviation of segments to form the anterior and posterior suckers is a new feature.

The nervous system may be considered under three heads:

(a) The central nervous system,

(b) The peripheral nervous system and

(c) The sympathetic nervous system.

(a) Central nervous system:

It consists of a pair of closely-connected cerebral ganglia in the fifth segments which lie above the roof of the pharynx. A triangu­lar sub-pharyngeal ganglionic mass is situ­ated in the fifth segment and lies beneath the pharynx. It has been formed by the fusion of four pairs of ganglia.

The cerebral ganglia and the sub-pharyngeal ganglionic mass are connected to each other by peripharyngeal connectives which form a stout ring round the pharynx. A ventral nerve cord runs along the mid-ventral line from the sub-pharyngeal ganglionic mass to the terminal ganglionic mass lying within the posterior sucker.

The ventral nerve cord in reality is a double chain closely apposed to each other and bears 21 fused ganglia. The first annulus of the seg­ments from sixth to twenty-sixth houses the ganglia. The terminal ganglionic mass is a large oval body formed by the fusion of seven pairs of ganglia. These seven segments constitute the posterior sucker.

(b) Peripheral nervous system:

Nerves given off from the ganglia of the central nervous system of different parts of the body form the peripheral nervous system.

Each cerebral ganglion gives a stout branch to the eye of its own side and also gives fine branches to prostomium and roof of buccal chamber.

The sub-pharyngeal ganglionic mass gives off four pairs of nerves which innervate the 2nd, 3rd, 4th and 5th pairs of eyes. Nerves from this mass also go to the floor of the buccal cavity, muscles of the body wall and segmental receptors of the first five segments.

Each paired ganglion of the ventral nerve cord gives off two pairs of nerves, the antero­lateral and postero-lateral. The anterolateral originates from the anterior part of the ganglion and continues through the first annulus to supply the body wall, sphincter around the nephridiopores and the receptors.

Near its origin it gives off a branch from which finer branches arise to supply the nephrid­ium, the vesicle and reproductive organs.

The postero-lateral originates from the ganglion and a little behind the anterolat­eral. It bifurcates into two branches. One branch runs vertically upward to innervate the viscera and mid-dorsal region of the body wall while the other branch runs posteriorly along the ventral body wall and supplies the testis sac, ventral part of the viscera and muscles.

(c) Sympathetic nervous system:

It is present in the form of nerve plexus lying beneath epidermis within muscle lay­ers of the body wall and on the wall of the alimentary canal. These plexuses are con­nected to the peri-pharyngeal nerve ring through scattered multipolar ganglionic cells.

11. Receptor Organs of Leech:

There are specially modified epidermal cells to serve as receptor organs (Fig. 17.39). These cells remain scattered and sunk within the body wall. In true sense, these cells cannot react themselves to the external stimuli but act as avenues of approach to the central nervous system.

The different receptors found in leech are:

(a) Eyes:

There are five pairs of eyes— one pair in each of the first five anterior segments. The first and second pairs of eyes are larger than the rest. The fifth pair is the smallest. The eyes are oriented in different directions. The first pair are directed for­ward, the second pair forward and outward, the third upward, the fourth backward and outward and the fifth backward.

Each eye is cylindrical in shape and the axis of the cylinder remains at right angle to the body surface. The broad outer surface of the eye is formed by a convex epidermal layer over which lies the cuticle (Fig. 17.39C). Both the cuticle and the epidermal layer are transparent. The rest of the cylinder is packed with cells arranged in longitudinal rows.

Each cell is large, clear and refractile and bears a cresentic hyaline substance, called lens or optic organella, at the centre. The cytoplasm is peripheral and the nucleus is round in these cells. The walls of the cylinder enclosing these cells are pigmented and are called the pigment cup. An optic nerve enters in each cylinder at its basal end and runs along the middle line.

The cells within the cylinder are photo­receptors. It is not known whether the eyes can form images of external objects. It is reasonable to assume that the eyes can dis­tinguish light from darkness and thereby can find out the direction or the source of light.

(b) Annular receptors:

Arranged in a line across the middle of each annulus of a seg­ment there are annular receptor organs (Fig. 17.39B). Each annulus bears eighteen such organs on its dorsal and eighteen organs on its ventral surface. These receptors are rep­resented as papillae projecting out of the skin and are formed by a group of flattened and overlapping cells. The receptors are tac­tile in nature.

(c) Segmental receptors:

These are present -as elevated and elliptical papillae on the first annulus of each segment of the body. Four pairs of segmental papillae occur on the dor­sal surface and three pairs on the ventral surface.

In each papilla there are ten to fifteen cells. The cells are elongated in shape and remain separated from one another by inter­cellular spaces. Each cell is provided with hair-like processes at its outer free end (Fig. 17.39B). The nucleus is oval and the cyto­plasm is finely granular in each cell. The cells are tactile in nature.

(d) Free nerve endings:

These are sim­plest of all receptors and consist of a tuft of delicate fibres scattered between the epider­mal cells of the body wall. These nerve end­ings help the leech to detect chemical changes in the watery medium in which it lives. Hence they are considered as chemoreceptors.

12. Reproductive System of Leech:

Hirudinaria is a hermaphroditic animal, i.e., both male and female reproductive or­gans occur in the same individual (Fig. 17.40). But they always practise reciprocal insemi­nation during copulation.

Male reproductive system:

It consists of:

(a) Testis sacs,

(b) Vasa efferentia,

(c) Vasa deferentia,

(d) Epidi­dymis,

(e) Ejaculatory ducts and

(f) Atrium.

There are eleven pairs of testis sacs. Each segment from twelve to twenty second houses a pair of sacs one on either side of the ventral nerve cord. The sacs are spherical in outline and whitish in colour. Being a part of the coelom the cavity is lined with coelomic epithelium and is filled with coelomic fluid. Spermatogonia or sperm mother cells are budded off from the inner walls of the sac.

These cells float in coelomic fluid and later on develop into sperms. The vasa deferentia are a pair of longitudinal ducts running forward from 22nd segment to 11th segment along the ventral body wall. They are slen­der and lie one on either side of the ventral nerve cord. The testis sacs and the vas def­erens of one side are connected by a very short and wavy duct, called vas efferens.

There are eleven pairs of vasa efferentia. The vasa deferentia become separate from the coelomic epithelial coverings in the middle of the 11th segment and form a pair of slender tubes. Each tube then turns inwards and becomes much convoluted to form the epididymis. A narrow and muscular ejacu­latory duct arises from the anterior end of each epididymis. The ejaculatory ducts open into the atrium.

The atrium is a pear-shaped organ lying mid-ventrally between 9th and 10th segments. It consists of an anterior swollen base, the prostate and a slender posteriorly directed neck—the penis sac.

The prostate houses several layers of unicellular glands, called prostate glands which pour their secretions through long ducts. The penis sac is muscular and bears a coiled and filamentous tube, called penis. The penis is eversible.

The sperms are carried to the epididymis through the two vasa deferentia and they remain stored there. From each epididymis sperms pass to the base of the atrium where they are bound together to form packets of spermatophores by the secretion of the pros­tate glands. The spermatophores are trans­ferred to the vagina of another leech during copulation through the tubular penis.

Female reproductive system:

The female reproductive system includes:

(a) Ovisacs,

(b) Oviducts,

(c) Common ovi­duct and

(d) Vagina.

There is only one pair of ovisacs situated in the 11th segment and very close to the ventral nerve cord. Each sac is coelomic in origin and contains a filamentous ribbon- shaped organ called ovary.

The ovary ap­pears as a nucleated cord of protoplasm with dilated ends. A short oviduct arises from the of ventral nerve cord, reproductive systems and a few base of each ovisac. The two oviducts join to form the, common oviduct in the 11th seg­ment.

The anterior part of the common oviduct is encased in a thick layer of albuminous glands. The glands are formed by cluster of cells which open into the common oviduct by individual ducts. The posterior part of the common oviduct takes a curved course and opens into the vagina.

The vagina is a pear-shaped muscular structure which remains in the posterior part of the 11th segment and opens to the exterior by the female gonopore situated on the mid-ventral line of the 11th segment. The inner walls of the vagina are thrown into folds.

The ova originate as buds from the wall of the ovaries and pass onto the common oviduct where they become wrapped in al­bumen and ultimately they come down to the vagina and remain stored there.

Copulation:

Leeches practise cross-fertilization. Dur­ing copulation two individuals come to­gether and become applied to each other in a head to tail position and as a result the male gonopore of one individual lies apposed to the female gonopore of the other and vice versa. Consequently the penis of one gets into the female gonopore of the other, and vice versa.

Consequently the penis of one gets into the female gonopore of the other, and there occurs a mutual exchange of spermatophores. The entire process conti­nues for about an hour and then the indi­viduals separate. Fertilization occurs inside the vagina. The usual breeding season is early spring.

Cocoon formation:

In the mean-time the glands of the clitellar segments secrete a snow-white foamy girdle around the clitellum. The leech then tries to free itself out of the girdle by wriggling.

During this activity fertilized ova are squeezed out into the jelly of the girdle. The leech finally frees itself out of the girdle and the two ends of the girdle are quickly plugged by the secretion of prostomial glands. And thus a cocoon is formed. Cocoon formation takes about six hours to become completed.

Structure of cocoon:

The shape of cocoon is like that of a barrel and it measures 30 mm in length and 15 mm in breadth. It is yellowish in colour and has a chitinous wall. The wall consists of an outer spongy layer and a tough inner layer. The narrow ends of the cocoon are provided with polar plugs with conical projections.

The spongy layer encloses a large number of air bubbles which keep the cocoon afloat on water. The cocoon is filled with albumen in which float the fertilized ova. The cocoon may house one to two dozens of such ova.

Development and emergence of young leeches:

Development is direct and occurs within the cocoon and the process is completed in about fifteen days. The albumen of the co­coon is used as food during development. When development becomes completed the polar plugs drop off and young leeches come out.