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In this article we will discuss about:- 1. Habitat of Leech 2. External Feature of Leech 3. Body Wall and Body Cavity 4. Locomotion 5. Alimentary System 6. Respiratory System 7. Vascular System 8. Excretory System 9. Nervous System 10. Reproductive System 11. Life-History.
Contents:
- Habitat of Leech
- External Feature of Leech
- Body Wall and Body Cavity of Leech
- Locomotion of Leech
- Alimentary System of Leech
- Respiratory System of Leech
- Vascular System of Leech
- Excretory System of Leech
- Nervous System of Leech
- Reproductive System of Leech
- Life-History of Leech
1. Habitat of Leech:
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The leech, Hirudinaria, is a segmented worm inhabiting freshwater tanks, ponds and shallow weedy lakes. They grow abundantly in swamps and pools near paddy fields, where they can easily suck blood from the bare-footed farmers as well as from the cattle. The animal is an ectoparasite.
The blood-sucking habit of the leech had been frequently utilised by the older physicians for the purpose of letting oat blood. This practice has now become obsolete and a leech is rarely employed for blood-letting.
2. External Feature of Leech:
The animal has a soft elongated body with a sucker at each end and when fully expanded it measures about 2.5 cms in length. It is flattened dorsoventrally but the dorsal surface is a bit more convex than the ventral. When contracted, the animal appears to be elliptical and does not exceed three or four inches in length. The skin is moist and slimy due to constant secretion of mucus, and bears beautiful colour patterns.
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The dorsal surface is dark-green and the ventral surface is pale yellow in colour. The two flanks are striped with black, yellow and orange bands. The entire body of a leech is divided externally into a large number of rings or annuli separated by transverse grooves.
There are more or less 102 such rings. These external rings, however, are only superficial structures and do not represent the actual number of true segments or somites in the body, the number of which is only 33.
A typical segment or metamere from the middle of the series includes five rings, of which the first bears the segmental receptor organs arranged in a ring round the body. The receptors are the sense organs of the leech and they appear as small black spots on the surface of the skin.
Each metamere bears four pairs of segmental receptors on its dorsal surface and three pairs on the ventral surface. The limits of the true segments are determined by the position, and a number of the segmental receptor organs and not on the basis of the superficial rings. It will be found that at both ends the body, a segment may be composed of only one or two rings.
There is an adhesive sucker at either end of the leech. The anterior sucker is composed of the first five segments. It is oval and cup-shaped, with its hollow towards the ventral surface. There are five pairs of prominent segmental receptors on the dorsal surface of the anterior sucker. These are the eyes of the leech.
The ventral hollow of the anterior sucker leads into the mouth. The anterior border of the sucker forms an upper lip which is highly sensory and bears the receptors for touch as well as taste. The posterior sucker is composed of the last seven segments. It is a muscular disc, circular in Outline, and is larger in size than the anterior sucker. It is used as an organ for adhesion and locomotion.
Apertures:
(1) The mouth is a triradiate opening situated ventrally in the centre of the cup-shaped anterior sucker.
(2) The anus is a small opening situated in the middle line on the dorsal surface of the 26th segment, in front of the posterior sucker.
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(3) The urinary openings are the seventeen pairs of minute nephridiopores on the ventral surface of the body. A pair of nephridiopores occur in each segment, beginning from the sixth and extending up to the twenty-second.
(4) Leech is hermaphrodite. The reproductive openings or gonopores are situated in the mid-ventral line, and are separated from one another by five rings,
(a) The male gonopore lies in the mid-ventral line of the tenth segment and a filamentous penis may occasionally be seen protruding through this opening,
(b) The female gonopore is situated in the mid- ventral line of the eleventh segment.
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A saddle-shaped clitellum is formed in summer during the breeding season of the leech. At this time the skin of segments 9th, 10th and 11th swells up and is changed in colour and texture. The clitellum produces a protective capsule or cocoon for the developing embryo.
3. Body Wall and Body Cavity of Leech:
The body wall of leech consists of a thin cuticle, the epidermis, the dermis, the musculature and the botryoidal tissue. It is best studied in a cross-section passing through the middle of the animal (Fig. 77).
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(1) The cuticle is a transparent non-cellular membrane which forms a delicate protective covering for the entire body. It is secreted by the underlying epidermis and is cast off periodically to be replaced by a new one.
(2) The epidermis is a single layer of columnar cells with their broad ends directed outwards. The spaces between the narrow inner ends of the epidermal cells are filled with a network of capillaries, a small amount of connective tissue and a large number of pigment cells, all of which are derived from the underlying dermis.
The epidermis, therefore, forms a vascular membrane through which exchange of gases during respiration may readily take place. Some of the epidermal cells are converted into pear-shaped glands which secrete mucus for moistening the skin; others are modified to form the various receptors which act as sense organs.
(3) The dermis is a thin basement membrane beneath the epidermis. It consists of fibres, capillary loops and brightly coloured pigment cells, which extend into the epidermis. The cuticle, epidermis and dermis together form the skin of the leech.
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(4) The muscles lie beneath the dermis. This constitutes the thickest part of the body wall. There is an outer layer of circular muscle fibres running round the body, and an inner layer of longitudinal muscle fibres extending along the length of the body.
Besides these, there are other bundles running obliquely and from dorsal to ventral surface. The muscles are composed of highly elastic spindle-shaped cells which help the leech to carry on locomotion.
The musculature encloses the body cavity which is filled up by a peculiar kind of spongy connective tissue called botryoidal tissue. With the help of this, the layer of longitudinal muscles is firmly adherent to the wall of the underlying alimentary canal.
The coelom or body cavity is distinct in a leech embryo. In the adult leech there is no distinct body cavity. The space is obliterated by the botryoidal tissue and reduced to certain longitudinal channels filled with blood. These are known as the hoemocoelomic channels.
The botryoidal tissue lies just beneath the longitudinal layer of muscles. It consists of a network of minute canals which are loaded with a dark brown pigment. The free ends of the canals communicate with the hoemocoelomic channels and they are filled with hoemocoelomic fluid.
Several canals are enclosed in a thin sheath of connective tissue which adheres to the gut internally and longitudinal muscles externally. The botryoidal tissue is believed to remove waste products but a definite proof in favour of this assumption is wanting.
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The gut lies in the centre enclosed by botryoidal tissue. It is represented in the middle of the body by the crop and its caeca. Four hoemocoelomic channels are found running longitudinally between the body wall and the gut wall.
Dorsal to the crop lies the dorsal hoemocoelomic channel, whilst the ventral hoemocoelomic channel, enclosing the nerve cord, is situated ventrally. Lateral to the caeca, there is a lateral hoemocoelomic channel on each side of the body.
4. Locomotion of Leech:
There are two principal methods of locomotion:
(1) Swimming, and
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(2) Crawling.
(1) The leech swims gracefully through water by undulating its body in a snake-like fashion. During swimming the dorsoventral muscles are tonically constricted, circular muscles are relaxed, whilst waves of contraction pass along the longitudinal muscles at the two sides of the body alternately.
(2) Crawling on a substratum is comparable to that of a looping caterpillar. The suckers play their part alternately as the organs for adhesion. Actual locomotion is effected by changing the length of the body. At first the posterior sucker is attached to the substratum. This produces a wave of contraction to pass forwards along the circular muscles, whilst the longitudinal muscles are kept in a relaxed state.
The result is the extension of the body forwards to its greatest length. The animal now fixes its anterior sucker upon the substratum. This produces contraction of the longitudinal muscles and simultaneous relaxation of the circular muscles.
The posterior sucker is released from the substratum and the contracting longitudinal muscles drag the posterior sucker forwards, whereby the body becomes arched and assumes the shape of a loop (Fig. 78—5). The posterior sucker is then fixed to its new position just behind the anterior sucker and the whole process is repeated over and over till the animal reaches its destination.
5. Alimentary System of Leech:
The alimentary canal is a straight tube and extends from the mouth in front to the anus behind. The triradiate mouth is guarded by a three-lipped partition called velum. In front of this, the hollow of the anterior sucker forms a pre-oral chamber.
The mouth leads into the buccal Cavity, lying just behind the velum. Embedded in the mucous membrane of the buccal cavity there are three cuticular jaws which can be moved by muscles. One of the jaws is mid- dorsal in position, and the other two are ventro-lateral.
The jaw is crescentic in shape and its bears a row of minute teeth on its free semicircular margin. On the sides of the jaw are numerous small button-like salivary papillae. The buccal cavity leads into an oval sac called pharynx which extends from the fifth to the eighth segment.
The pharyngeal wall is supported by well-developed muscles and the interior of the pharynx contains merable salivary glands. The ducts of the salivary glands opens on the salivary papillae. The pharynx leads into the crop through a short and narrow tube, the oesophagus.
The crop extends from the 9th to the 18th segment and consists of ten thin walled chambers, each of which is provided with a pair of lateral pockets or caeca. The chambers of the crop and the caeca associated with them are serially arranged. The caeca increase in size from before backwards. The first pair are the smallest and the last pair are the largest.
All the caeca point backwards and the last pair of caeca may run as far back as the 22nd segment or even beyond—one lying on either side of the intestine. In a recently fed specimen the crop and the caeca are found engorged with blood which has been sucked out of the victim. The crop communicates through a narrow sphinctered opening with a small stomach situated in the 19th segment.
The stomach passes imperceptibly into a straight narrow tube, the intestine which extends from the 20th to the 22nd segment. The posterior end of the intestine is constricted and becomes continuous with the rectum which is a thin-walled tube opening at the anus, in the mid-dorsal line of the 26th segment.
Feeding and Digestion:
The leech is a sanguinivorous animal. It sucks blood from vertebrates like man and cattle. A leech attaches to its victim by the posterior sucker. The cup-shaped anterior sucker is then firmly applied to a suitable part of the skin.
The pre-oral chamber flattens out and the jaws are protruded with their serrated margins pressed against the skin. The jaws are moved together by powerful muscles so as to make a triradiate wound, from which blood oozes out freely and passes into the buccal cavity. The blood is sucked into the crop by alternate contraction and expansion of the muscular pharynx.
The process is similar to pressing and releasing the rubber teat of a dropper for sucking ink to fill a fountain pen. Meanwhile, the blood is mixed with saliva containing hirudin. Admixture with hirudin prevents coagulation, so that liquid blood passes easily into the crop and caeca which act as a huge reservoir.
The animal sucks three or four times its own weight of blood at a single meal. After this it remains inactive in a hiding place for several months. The blood which is stored in the crop is converted into a jelly-like mass. The erythrocytes swell up and disintegrate and their haemoglobin comes into solution in the blood plasma.
The animal is very economical as regards the expenditure of its food store. A drop or two of the blood may pass slowly through the sphincter into the stomach, where it is digested and changed into a green mass by the action of proteolytic enzymes.
The digested food is absorbed by the folds of the intestine and assimilated. The residue passes into the rectum and voided as a black mass through the anus. A full meal requires about one year to be digested and absorbed.
6. Respiratory System of Leech:
The skin forms a large respiratory surface. The cutaneous respiration is facilitated by the secredon of mucus which keeps the skin in a moist slate. The skin is profusely supplied with capillaries which penetrate between the epidermal cells.
The one-layered epidermis serves as a permeable membrane through which oxygen dissolved in the water is taken in and the carbon dioxide circulating in the hoemocoelomic fluid is given out.
7. Vascular System of Leech:
There are no true blood vessels in the leech. As substitutes for blood vessels, there are coelomic channels filled with hoemocoelomic fluid. The hoemocoelomic fluid is tinged red with dissolved haemoglobin and contains amoeboid leucocytes. It circulates through four hoemocoelomic channels which rim along the length of the leech.
There is a dorsal channel above the alimentary canal, a ventral chamber below the alimentary canal, and two lateral channels, one on each side of the alimentary canal. They meet one another in the posterior sucker where the ventral channel dilates into a sac to receive the others.
There is no heart for propelling the hoemocoelomic fluid but the lateral channels, having muscular wall and being contractile, perform the function of the heart. Moreover, the presence of valves within the lateral channels prevent back flow.
The dorsal and ventral channels are thin-walled and non-contractile. They serve mainly as distributing channels. The lateral channels, on the other hand, serve both as distributing and collecting channels.
The hoemocoelomic fluid flows from behind forwards in the dorsal and lateral channels; but in the ventral channel it flows backwards. The dorsal channel mainly supplies the alimentary canal and the dorsal body wall. The ventral channel supplies the nephridia and the ventral body wall. The lateral channels supply the nephridia and the reproductive organs.
8. Excretory System of Leech:
Excretion is mainly effected by a series of glandular bodies tailed nephridia. There are 17 pairs of nephridia—the first pair lying in the sixth segment and the seventeenth pair in the twenty- second segment.
The nephridia have no internal openings but they open externally by 17 pairs of nephridiopores arranged metamerically on the ventral surface of segments VI to XXII. The nephridiopores are the so-called urinary openings.
A typical nephridium, from the middle of the series, is a horse-shoe-shaped mass with two limbs. The main bulk of the nephridium constitutes the main lobe. It is situated ventrally on the outer side of the lateral hoemocoelomic channel. Of the two limbs of the main lobe, the anterior is longer than the posterior. The posterior limb of the main lobe is continued forwards to join a stout apical lobe.
A narrow inner lobe extends between the two limbs of the main lobe fitting into the concavity of the horse-shoe- shaped mass. Twined and coiled round the apical lobe, there is a slender cord-like initial lobe which extends from the posterior limb of the main lobe to the nearest testis-sac.
The ventral hoemocoelomic channel gives out branches which form a dilated ampulla on the wall of the testis-sac. The initial lobe of the nephridium ends blindly over the outer surface of this ampulla.
Inside the ampulla, there is a peculiar structure called ciliated organ, which corresponds to the nephridial funnel in other annelid worms. But the ciliated organ in leech has lost all connection with the nephridium and takes no part in the extraction of waste products.
A tube called vesicle-duct arises from the inner end of the anterior limb. This tube turns backward to join a thin-walled pyriform bladder called vesicle. The bladder communicates with the corresponding nephridiopore on the ventral surface of the animal by a short ureter.
There is no testis-sac in segments VI to XI. Consequently the initial lobes of the first six pairs of nephridia end blindly in the connective tissue, by the side of the ventral nerve cord. These are the pretesticular nephridia. The following eleven pairs of nephridia are connected to the testis-sacs by their initial lobes and are therefore known as the testicular nephridia.
A nephridium is a mass of loosely packed glandular cells. There are minute intracellular spaces which join with one another to form fine canaliculi. The vesicular duct arises by the union of these canaliculi at the anterior limb of the main lobe.
The nephridia are profusely supplied with branches from ventral and lateral hoemocoelomic channels. The glandular cells extract excess of water and nitrogenous waste products by a process of selective filtration. These pass by the vesicular duct to the bladder and thence eliminated through the nephridiopore.
The botryoidal tissue filling up the body cavity is believed to be accessory excretory structures.
9. Nervous System of Leech:
The nervous system consists of three parts—the central nervous system, the peripheral nervous system and the sympathetic nervous system.
The central nervous system is enclosed within the ventral hoemocoelomic channel and lies ventral to the gut. It consists of a pair of supra-pharyngeal ganglia, a pair of peripharyngeal connectives, a sub-pharyngeal ganglionic mass, and a ganglionated ventral nerve cord with a large terminal ganglionic mass at the posterior end of the body,
(a) The supra-pharyngeal ganglia or brain lie close to one another on the dorsal surface of the pharynx in the fifth segment,
(b) The peripharyngeal connectives, one on each side, form a stout nerve collar surrounding the anterior part of the pharynx in the fifth segment. They connect the supra with the sub-pharyngeal ganglionic mass,
(c) The sub-pharyngeal ganglion is situated ventral to the pharynx in the fifth segment. It consists of several ganglia fused together.
(d) The ventral nerve cord connects the sub-pharyngeal ganglionic mass, in the fifth segment, to the terminal ganglionic mass, lying in the middle of the posterior sucker. Throughout its whole course it is situated in the mid-ventral line.
The cord bears 21 ganglia, one in each segment, from the sixth to the twenty-sixth. The ventral nerve cord and the ganglia associated with it are really double and consists of two longitudinal chains closely opposed to one another.
(e) The terminal ganglionic mass, in the middle of the posterior sucker, is a large oval body formed by the fusion of the last seven pairs of ganglia.
The peripheral nerves are paired structures arising from the ganglia. The supra-pharyngeal ganglia supply nerves to the first pair of eyes, to the anterior lip and the roof of the buccal chamber. The sub-pharyngeal ganglionic mass innervates the remaining 4 pairs of eyes and the muscles of the body wall of the first five segments.
Each segmental ganglion supplies two pairs of lateral branches, which are distributed to the muscles of the body wall and the segmental sense organs. The terminal ganglionic mass sends out branches to the various structures of the posterior sucker.
The nerves are of two kinds:
(1) Afferent or sensory which connect the receptor organs to the central nervous system,
(2) Efferent or motor which connect the central nervous system with the effector organs, that is, muscles and glands.
The sympathetic nervous system is poorly developed. It consists of an extensive but delicate nerve plexus lying on the dorsal wall of the alimentary canal and supplying the involuntary muscles throughout the body.
Receptor Organs of Leech:
There are 4 kinds of receptors in the leech. These are composed of modified epidermal cells connected with sensory nerve endings.
(i) The simplest receptors are the free nerve endings found scattered everywhere on the surface of the animal. They occur as tufts of delicate fibres between the epidermal cells, and help the leech to detect changes in the quality of the surrounding water.
(ii) The annular receptors occur as small papillae arranged in a ring round each annulus of a segment. They are probably concerned with the sense of touch.
(iii) The segmental receptors are found as raised dots round the first annulus of each segment throughout the body. Most of these are tactile organs but there are a few which carry on light- perceiving function.
(iv) There are five pairs of eyes which occur as black dots on. the dorsal surface of the anterior sucker. Each eye is cylindrical in shape, and consists of a number of refractive cells arranged and to end in a longitudinal rows. Each cell is a photo-receptor and contains a crescentic lens surrounded by a thin layer of cytoplasm and a small nucleus.
The photo-receptor cells are enclosed in a thick black-pigmented sheath. The top of the eye is covered by a cap of transparent epidermal cells. A nerve enters each eye at its base and distributes branches to the photo-receptor cells. The eyes can extinguish light from darkness. They can also locate the source of light, but most probably they cannot form images of external objects.
10. Reproductive System in Leech:
The leech is monoecious or hermaphrodite, that is, both male and female reproductive organs are found in the same individual
Male reproductive organs consist of:
(1) Testis-sacs,
(2) Vasa efferentia,
(3) Vasa deferentia,
(4) Epididymis,
(5) Ejaculatory ducts, and
(6) Atrium.
There are eleven pairs of testis-sacs, one pair in each of the segments XII to XXII. These are situated ventral to the alimentary canal, between the nerve cord and the nephridia. A testis- sac is whitish in colour and spherical in shape. Being a part of the coelom, its cavity is filled with coelomic fluid. Each testis-sac gives off laterally a short narrow duct called vas efferens.
All the eleven vasa efferentia of one side open into a long wavy tube known as vas deferens. Beginning in the twenty-second segment, the two vasa deferentia run forwards on either side of the nerve cord. At the posterior end of the tenth segment each vas deferens becomes greatly convoluted to form a compact mass called epididymis.
A narrow but muscular ejaculatory duct comes out from the anterior end of each epididymis. The right and left ejaculatory ducts lead inwards to open into a median sac called atrium. The atrium is a pear-shaped muscular organ lying anteriorly in the mid-ventral line of the tenth segment.
The atrium consists of two parts: a broad anterior part containing the prostate glands, and a narrow posterior part or penis-sac containing a thin eversible penis which is capable of being protruded through the male gonopore on the mid-ventral line of the tenth segment.
Sperm mother cells are budded off from the inner wall of the testis-sacs and develop into spermatozoa. The sperms are carried along the vasa deferentia to be stored within the epididymis.
They pass into the base of the atrium where they are glued together to form packets or spermatophores by the sticky secretion of the prostate gland. The spermatophores are transferred during copulation into the female gonopore of another leech through the narrow canal of the tubular penis.
The female reproductive organs consist of a pair of ovisacs, a pair of oviducts, the common oviduct and the vagina. There is only one pair of ovisacs situated in the eleventh segment. The ovisacs are coelomic in origin and each contains a coiled ribbon-shaped ovary with dilated ends.
The base of each ovisac is continued as a short and narrow oviduct. The two oviducts unite in the mid-ventral line of the eleventh segment to form a common oviduct. The anterior part of the common oviduct is surrounded by albumen glands which open into it.
The posterior part of the common oviduct takes a curved course to open into the vagina. The vagina is a pear-shaped muscular bag in the posterior part of the eleventh segment.
Its narrow neck leads into the female gonopore which is situated on the mid-ventral line of the eleventh segment. The ova are budded off from the ovaries. They receive a coating of albumen as they pass through the common oviduct into the vagina where they are stored in readiness for fertilization.
11. Life-History of Leech:
The usual breeding season is early spring. Two worms meet and become applied to each other in a head-to-tail position, so that the male gonopore of one lies opposed to the female gonopore of the other. There is a mutual exchange of spermatophores between the two worms and thus cross fertilization is ensured.
Mating may take place in moist land or under water and the whole process usually lasts for an hour, after which the worms separate. The spermatophores received from the other worm pass into the vagina, where fertilization of the ova takes place.
In the meantime the clitellar segments (9th, 10th and 11th) swell up and secrete a snow-white frothy substance which gradually sets into a jelly-like girdle. The leech now tries to wriggle out and free itself from the soft mass. During the struggle fertilized ova are discharged through the female gonopore, coming to lie in the jelly.
The leech ultimately frees itself from the girdle, the two ends of which are quickly plugged. The soft mass, on exposure to air, hardens to form a cocoon which contains fertilized ova floating in the albumen secreted by the clitellar glands. The cocoon formation is completed in about six hours, after which they are laid in moist places by the side of a pool.
Each cocoon is a barrel-shaped structure about 30 mm. long and half as broad. It is light yellow to amber in colour and is provided with a hard chitinous wall. A cocoon contains about two dozen embryos.
Development takes place within the cocoon directly, and young leeches hatch out in about a fortnight. The polar plugs give way to let out the young, which in the meantime have completely used up the albumen. There is no metamorphosis.
