Nervous system of Leech

The nervous system of Leech

The nervous system of Leech is basically annelidan but it shows advancement over the other annelids. It shows central, peripheral and autonomic nervous systems.

 

1. Central nervous system:

It is enclosed in the venteral haemocoebmic canal. It shows.

 

i) Cerebral ganglia and nerve ring

ii)Nerve cord

iii) Terminal ganglionic mass

The ganglia are made by nerve cells.

 

i) Cerebral ganglia and nerve ring: On the pharynx in the fifth segment a pair of cerebral of supra' pharyngeal ganglia are present. They are fused. It is called brain.

Below the pharynx in the fifth segment sub-pharyngeal ganglionic mass is present. It is formed by the union of the 4 pairs of ganglia.

The sub-pharyngeal ganglionic mass and cerebro (or) supra pharyngeal ganglionic mass will be connected by two circumpharyngeal connectives. Thus, a nerve ring is formed around the pharynx.

 

 

 

Hirudinaria-nervous system

 

ii) Nerve cord: From the subpharyngeal ganglionic mass the nerve cord

starts. It runs backwards. It is mid ventral in position. It is present below the

posterior sucker. It wjll join the terminal ganglionic, mass present near the

posterior sucker.

 

The nerve cord shows 21 ganglia, one ganglion in the first annulus of each segment from 6th to 26th segments of the body: Each ganglion is formed by the union of 2 ganglia. They give nerves or peripheral nerves. The nerve cord is made by nerve cells and nerve fibres.

 

iii) Terminal ganglionic Mass: In the posterior sucker of leech

terminal ganglionic mass is present. It is the fusion product of 7 pairs of ganglia.

 

 

2. Peripheral nervous system:

From the ganglia of the central nervous system many nerves will arise. These will constitute the peripheral nervous system.

i) One pair of nerves will arise from the front side of brain and go to first pair of eyes and prostomium.

 

ii) Four pairs of nerves arise from the sub-pharyngeal ganglionic mass. They go to 2, 3, 4, 5th pairs of eyes.

iii) Two pairs of nerves arise from each ganglion, and go to the body parts.

iv) Seven pairs of nerves will arise from terminal ganglionic mass and they go to posterior sucker.

External Morphology of Leech

 

The leech is an ectoparasite. The leech, Hirudinaria, is a segmented worm found in  fresh­water tanks, ponds, and shallow weedy lakes.   Leech can easily suck blood from farmers as well as from the cattle. Such type of feeding habit is called a sanguivorous feeding habit.

The blood-sucking habit of the leech had been used  by the older physicians for the purpose of treating old wounds. Hence it is also called as medicinal leech..

 

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 dorso-ventrally 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.

The dorsal surface is dark-green  or olive green in colour and the ventral surface is pale yellow in colour.. The entire body of a leech is divided externally into a large number of rings or annuli separated by transverse grooves.

The body of leech consists of 33 segments.

 

 Metamerism in leech

          When segmentation in bilateral animal such as annelids involves a longitudinal division of body into a linear series of similar section of parts is termed as metamerism. Or  metameric segmentation.

Body of annelids is metamerically segmented. Internal and external metameism is visible in most of the annelids.  The body of leech is divisible into many regions as follows.

1.      Cephalic region                     I-V

2.      Pre-clitellar region               VI-VIII

3.      Clitellar region                    IX-XI

4.      Middle region                      XII-XXII

5.      Caudal region                      XXII-XVII

6.      Posterior sucker                   XXVIII-XXXIII

 Each segment is divided by ring also called as annuli. The segment is either uni-annulate, bi-annulate, tri-annulate or Quinqua-annulate. Segment with five annuli is called quinqua -annulate segments or complete segment. The distribution of annuli in different regions of body is as follows.

 

Sr. No.	Body region	Segment No.	Distribution of annuli	Complete or incomplete
1	Cephalic region	I-V	I and II         Uni-annulate	Incomplete
			III -                 Biannulate	Incomplete
			IV and V       Tri-annulate	Incomplete
2	Pre-clitelar Region	VI-VIII	VI-       Tri-annulate	Incomplete
			VII and VIII Quinqua-annulate	complete segment
3	Clitelar region	IX I XI	Quinqua-annulate	complete segment
4	Middle region	XII-XXII	Quinqua-annulate	complete segment
5	Caudal region	XXIII- XVII	XXIII           Tri-annulate	incomplete segment
			XXIV and XXV     Bi-annulate	incomplete segment
6	Posterior sucker	XXVIII – XXXIII	Uni- annulate.	incomplete segment

 

 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 first annuls marks the boundry of the segment.

To both the end of body there is presence of sucker.. the anterior sucker is oval and cup like.  The anterior sucker consists of first 5 segments. On dorsal surface of anterior sucker, there are presence of five pairs of prominent eyes.

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.

(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.

(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.

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

 

The Haemocoelomic system of Leech

 

The Haemocoelomic system of Leech

consists of :

1) Four longitudinal haemocoelomic channels.

2) Segmental branches.

3) Capillaries.

4) Haemocoelomic fluid.

 1. Longitudinal Haemocoelomic Channels :

Out of four one is dorsal, one is ventral and two are lateral in position.

 i) Dorsal canal: It lies above the alimentary canal and extends the entire length. '

 a) It is a distributing channel. In each segment it gives two pairs of dorso laterals from its ventral side. They break into capillaries in the dorsal body wall.

 b) It gives dorso-intestinals to the alimentary canal all along its length.

 c) In the sixth segment, it breaks into branches which form capillaries.

 d) In the 22nd segment it bifurcates into branches. They open into ventral canal at the end.

 e) The haemocoelomic fluid flows forwards in the dorsal channel,

  

Haemocoelomic system of a single segment in lateral view.

 

 ii) Ventral Channel: It lies below the alimentary canal and extends the entire length. It encloses the ventral nerve cord. It enlarges at the anterior and posterior ends to surround the nerve mass. Ventral channel is a distributing channel.

a) In each segment, it gives of a pair of cutaneous branches. Each cutaneous branch divides into two smaller branches. They end in capillaries in the ventrolateral and dorsolateral body wall.

 b) From 12 to 22 segments ventral channel gives a pair of nephridial branches. It ends in capillaries in nephridium and body wall.

 

iii) Lateral channels : These are two in number which are placed on either side of the alimentary canal they are muscular and valvular. In these channels the haemocoelomic fluid flows from behind forwards. They are both distributing and collecting channels. In the 6th segment they break into capillaries.

 In each segment both the lateral channels receive laterodorsal and latero-laterals. In each segment they give lateroventral branch.

 i) Latero-lateral : It starts from the lateral part of the body wall and nephridium and unites which lateral channel.

 ii) Latero-dorsal: It arises from nephridial wall and dorso lateral parts of the body wall. It is big. It unites with lateral channel. In each segment two latero-dorsal opposite sides are united by a transverse loop. They are present from 6 to 22 segments.

 iii) Latero ventral: It starts from the inner side of the lateral channel and supplies blood to nephridium and ventro lateral parts of the body wall. This branch divides into anterior and posterior branches. They unite with the same branches of the opposite side and from ventral commissures. They are one in each segment from 6 to 23 segments.

 Latero-lateral and latero-dorsal canals are collecting canals. But Latero-ventral is distributing canal.

 

 HAEMOCOELOMIC SYSTEM In Posterior sucker

 

 Capillaries :

 All the longitudinal channels and their branches divide into capillaries. They are three 3 sets.

 i) Botryoidal capillaries : They surround the viscera.

 ii)Intermediate capillaries : They penetrate the body muscles.

 iii) Superficial capillaries : They enter into the epidermis.

 iv) Haemocoelomic fluid : It is a coelomic fluid. It is red in colour because haemoglobin is dissolved in it. This is called blood like fluid. Circulation :

 In the haemocoelomic system the red haemocoelomic fluid flows in a definite course. Dorsal, ventral and the two lateral haemocoelomic channels are united in the 26th segment.

 a) The dorsal and ventral haemocoelomic channels distribute the fluid to body parts.

 b) The two lateral channels are distributory as well as collecting channels.

1) The dorsal channel gives the following.

 a) Dorso-lateral vessel which supplies blood to dorso-lateral body wall.

 b) It gives dorso-intestinal vessel which supplies fluid to gut wall.

 2) The fluid from dorso-lateral body wall is pushed into ateral channel through latero-dorsal vessel.

 3) The blood from gut wall is carried to lateral channel through latero-dorsal vessel.

4) The lateral channels collect fluid from nephridia and gonads through latero-lateral vessels.

 5) Ventral channel supplies blood to nephridia and gonads.

 6) Lateral channels distribute blood to gonads, nephridia, body wall and gut wall through ventral comrnisure.

 Thus, the blood is in circulation in the Haemocoelomic system of leech.

Leech (Hirudinaria ) Reproductive system

 

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.

  

Testis-sacs:  

There are about 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.  Each testis sac is rounded and  whitish in colour.   Its cavity is filled with coelomic fluid.

Function. Production of spermatozoa

 

Vas efferentia:

It is in the form of thin delicate duct arised from each testis-sac gives off  laterally narrow duct called vas efferens.

 Function – Carries sperm to vas deferens

 

Vasa deferentia:

All the eleven vasa efferentia of one side open into a long tube known as vas deferens. Running from either side of ventral nerve cord from  twenty-second segment up to the 10th segment.

 Function: transfere of sperm into epididymis

 

Epididymis.

In tenth segment vas deference  become coiled or convoluted to from a compact mass called epididymis.

Function: storage of sperm

Ejaculatory duct

It is short and  narrow duct arrsed from the anterior end of each epididymis. The right and left ejaculatory ducts lead inwards to open into a median sac called atrium.

Function : transfer of sperm into atrium

 

Atrium:

 It is a pear-shaped muscular organ lying anteriorly in the mid-ventral line of the tenth segment. It consist of two parts i.e anterior prostate chamber and posterior narrow penis sac. The sac encloses eversible penis.  

Function:  formation of sperm bundles or sperm pocket.

 

Male genital aperture : it is in the from of minute aperture opens in tenth segment mid-ventrally as male genital aperture.

 Function: Deposition of sperm into the female genital aperture of other leech.

 

 Female reproductive system.

 1.     Pair of ovisacs

2.     A pair of oviducts

3.     Common oviduct

4.     Vagina

 Pair of ovisacs

 In leech  only one pair of ovisacs situated in the eleventh segment.

Each ovisac is oval and rounded structure encloses the cavity called coelomic cavity filled with coelomic fluid. In coelomic fluid  a coiled ribbon-shaped ovary with dilated ends is floats. .

 

A pair of oviducts: from each ovisac arises a short and narrow delicate duct called oviduct. The

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.

Common oviduct:

The two oviduct from either side unite together to from common oviduct. The common oviduct  turned  in to ‘S’ shaped  curve to open into the vagina. At the junction of common oviduct there is presence of whitish mass called albumen glands which open into it.

Vagina:

 It is a pear-shaped muscu­lar bag  like structure  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.

 Reproduction Organs of Hirudinaria in X to XIII segments

 

 

Copulation and FORMATION OF COCOON or Ootheca

 

After copulation leech will develop a ootheca around the clitellar zone. The completely formed cocoon is laid on moist soil. In this cocoon the zygotes will develop into young individuals.

 

Copulation:

In the months of March and April leeches will show copulatory process. Two mature leeches living near by will come together and meet on their ventral surfaces in opposite directions. The male genital opening of one leech lies opposite to the female genital opening of other leech. The penis of one leech will penetrate into the female into the genital opening of the other leech and deposit spermatophores in the vagina. Thus, both leeches will exchange their sperms during copulation. The entire process takes nearly one hour. It takes place on land or in water.

 

Cocoon or ootheca : In leeches the cocoon or ootheca will be formed after copulation. The formation of cocoon is very slow process. It takes nearly six hours’ time. Clitellum develops in leeches in the 9th to 11th segments region which is responsible for the formation of cocoon.

 

The clitellar glands will secrete a white girdle around clitellar region. In addition to this the clitellar glands will deposit albuminous fluid in the cocoon or ootheca. After the formation of ootheca the fertilized eggs are laid in the cocoon through the female genital openings present in the ventral side of the body in between 2nd and 3rd annulus of 11th segment. By rhythmic contractions of the body leech withdraws its body from the cocoon. Both ends of the cocoon will be closed by plugs. These plugs are secreted by the prostomial glands. Thus a box like ootheca or cocoon is formed. It is barrel shaped. It is laid on moist land. It is 25-30 mm in length and 12-15 mm in width. When it is exposed to air it gets hardened.

 Within the cocoon or ootheca the development of the zygote takes place. Development: The development of zygote takes place in the cocoon.

 1) This development takes nearly 15 days.

 2) The cleavage is unequal.

 3) The developing embryo absorbs albumen as its food.

 4) The development is direct and larval form is absent.

 One to twenty four leeches may come out of each cocoon. The young one resembles the adult.