REPRODUCTION IN PARAMECIUM

In case of paramecium, the process of reproduction is of the following types:-

1. Asexual method
2. Sexual method

 

ASEXUAL METHOD:-

In case of asexual method, reproduction in paramecia reproduces by transverse binary fission. This occurs in favorable conditions, i.e., under good condition of food and water etc. The division of the body is transverse i.e., at right angle of the longitudinal axis of the body.

 

• Firstly, the paramecium stops the feeding and its oral groove disappears.
• The micronucleus divides by the complicated process of mitosis, during nuclear division. The nuclear membrane remain intact, the two daughter nuclei towards the opposite ends of the cell.

• 36     o 50 chromosomes appears during mitosis (depending upon the race).
• Macronucleus divides a mitotically by simple elongation of the nucleus and constricted off in the middle.
• In the same time when macro nucleus divides, a transverse constriction forms around the middle of the body.
• The furrow continues to deeper and deeper ultimately dividing the cytoplasm into two halves or daughter paramecia.
• The anterior part called porter and posterior opisthe, before the separation of each half, new parts develops i.e., necessary for complete paramecium.
• Each daughter receives one contractile vacuole form the parent and second one is formed. Oral groove develops in each and buccal structures again constituted. These two daughter paramecia are equal in size with complete organelles. These grow to full size before dividing again.
• The process is completed 20 to 120 minutes depending upon condition of food and temperature. It may occur 2 to 3 times a day in Paramecium candatum and 5 times in paramecium aurelia.

Sexual reproduction takes place by the following methods

CONJUGATION:-

This process is given by the HERTAIG and MAUPAS (1889).

Conjugation is the simple temporary union of two individuals of one and the same species for the purpose of exchanging the part of nuclear material.

Two individual comes in contact ventrally by their oral grooves and attached till the end of process. Pellicle and endoplasm degenerates and form protoplasmic bridge between the two individuals.

Each individual called conjugate or gametocytes. The conjugating pair swim actively. Then a sequence of complicated nuclear change takes place in each conjugate in the following manner-

• Macronucleus breaks up into fragments and lastly absorbed by the cytoplasm.
• The micronucleus of each divides twice, forming four (4) Haploid micronuclei. One division is reduction division.
• Out of four, three degenerates in each conjugation and disappears. Remaining one micronucleus divide unequally forming two pronuclei or gametic nuclei (Genetically identical).
• Out of two, one is smaller is called active male pronucleus and the larger is called stationary female pronucleus in each conjugate.
• The male pronucleus passes from one conjugate to the other through protoplasmic bridge into the female pronucleus.
• Now the fusion of male and female pronucleus takes place forming a diploid zygote nucleus or Synkaryon or amphinucleus. The fusion of two nuclei from two different individuals is called Amphimixis.
• The two paramecia now separate after 12-48 hours and are now called as ex-conjugates. In each ex-conjugate, zygote nucleus divides 3 times forming 8 nuclei.
• Of these four become macronuclei and four become micronuclei.
• Three micronuclei disintegrate and disappears, one remaining micronucleus divides with the binary fission of each ex-conjugates.
• Thus, two daughter paramecia are obtained having two macronuclei and one micronucleus.
• The daughter paramecia again divide, with division forms two individuals. Each containing one macronucleus and one micronucleus.
• Thus, each conjugate produces four daughter individuals.

AUTOGAMY OR AUTOMIXIS :-

Autogamy resembling conjugation, Diller 1936 reported this type of reproduction in Paramecium aurelia. Autogamy takes place in a single paramecium.

 Diller called the autogamy as self-fertilization.

•  Two micronuclei of the same organism fuse together forming a Synkaryon.
• During the process in P. aurelia, the macronucleus grows into an irregular mass and breaks into pieces and lastly disappears.
• The two micro -nuclei divide twice, once meiotically to form 8 haploid nuclei.
• Out of 8, 7 nuclei degenerate and one divide to form 2 sister gamete nuclei. These two are homologous to the stationery and migratory pronuclei.
• These enter a protoplasmic cone, temporarily formed near cytostome and then fuse together to form a zygote nucleus or Synkaryon.
• Synkaryon divide twice two form four nuclei, two of which macronuclei and two become micronuclei.
• The Cell body divides and 2 micronuclei also divides to form 4 nuclei, 2 macronuclei as such.
• Paramecium divides into two daughter individuals   having one macronucleus and two micronuclei.
• The process is completed In about two days involving a readjustment   of the nucleo-cytoplasmic relationship. 

 

CYTOGAMY :-

Cytogamy recently reported by WICHTERMAN in 1940 in P. caudatum. Cytogmy is a process without nuclear exchange. The process resembles conjugation in that two small paramecia (200 µ long) fuse together by their oral surfaces.

• The early nuclear division are similar to the conjugation but there are no nuclear exchange between the two individuals.
• The two gametic nuclei in each parent are said to be fused to form a synkaryon as in autogamy. This process is completed in about 13 hours.
•  The genetical consequence of cytogamy is always homozygocity.

 

ENDOMIXIS :-

 Endomixis is an interesting phenomenon involving total internal nuclear reorganization within a single individual.

WOODRUFT and ERDMANN 1914 reported in bimicronucleate species P. aurelia. It is occurring periodically at regular interval of about 30 days in P. caudatum and 16 days in P. aurelia. It occurs as follows-

 

• Micronucleus disintegrates and disappears.
• Two micronuclei divide twice forming 8 daughter nuclei.
• Out of 8, 6 degenerates.
• The paramecium divides, each individual getting one micronucleus, which divide twice forming 4 nuclei.
• Two nuclei become macronucleus and two micronuclei.
•  Paramecium with 2 macro and 2 micro nuclei divides and at the same time 2 micronuclei divides.
• Two daughter cells obtained, each with one macro nucleus and two micro nuclei. In the end, four daughter cells are formed from single individual.

 

HEMIXIS :-

This process is pointed out by DILLER 1936. It is a simple method of nuclear exchange different from conjugation, binary fission and autogamy in single individual.

During these changes termed Hemixis by DILLER.

• The micronuclei do not participate.
• But the macronucleus throws out 2 to many equal or unequal fragments of chromatin balls (DNA) to be absorbed in the cytoplasm.
• Some of the macronuclei, they behave normally at the cell division.
• The significance of hemixis is not well understood.

 

ENCYSTMENT

 Encystment occurs in green species (P. bursaria). It has not been seen except Michelson’s on P. caudatum. The cysts are said to be looks like sand grains. 

                                      

REPRODUCTION IN PROTOZOA

Reproduction is the process by which living organism produce young ones of their own species.  Reproduction is defined as behavioral instinct of living organism to maintain the race.

In protozoa reproduction takes place by:-

1. Asexual method
2. Sexual method

1.       Asexual method of reproduction in protozoa:-

• Many Protozoa reproduces by Asexual method, in which single parent is involved.
• The development of new individuals without the fusion of male and female gametes.
• Asexual reproduction may be of following types-

1. Binary fission and multiple fission
2. Budding
3. Plasmotomy

 

BINARY FISSION IN PROTOZOA:- 

It is most common in protozoa. During binary fission animal body divides in such a place that two identical halves are produced.

 

Simple binary fission or orthodox type:-

This simple binary fission is found in irregular shaped organism like – amoeba. Plane of division is unknown or division is in any plane. Nucleus divides first than cytoplasm divides like mitosis.

 Transverse binary fission:-

Transverse binary fission is found in Paramecium (ciliates).

Plane of division is always transverse to the longitudinal axis of the body of organism. It takes place during favorable conditions; when food, temperature and water is suitable.

• Micro nucleus divides into two nuclei by process of mitosis.
• Mega or Macro nucleus divides by Amitosis.
• Micro and Macro nuclei moves towards opposite pole.
• Constriction appears in the middle.
• One contractile vacuole goes to one daughter paramecium, other is reformed.
• Parental oral groove, vestibule, cytopharynx goes to one daughter paramecium but in other daughter paramecia, new set of these structures are formed.
• Binary fission takes 30-120 minutes to complete the process.

 

Longitudinal binary fission:-

Longitudinal binary fission is found in certain ciliates and flagellates. e.g., Vorticella, Euglena.

In longitudinal binary fission, nucleus and cytoplasm divides in longitudinal plane.

Oblique binary fission:-

In this type of binary fission, organism divides by oblique plane. Oblique binary fission takes place in dianoflagellates.

 

MULTIPLE FISSION IN PROTOZOA :-

In multiple fission, the nucleus of the cell divides very rapidly into many nuclei. Each daughter nucleus gradually surrounded by little mass of cytoplasm and forming pseudopodia spores, as in Amoeba. In case of malarial parasite it forms schizont.

Multiple fission is found in most protozoans like – Amoeba, Plasmodium and monocysts etc. By the rupture of cell membrane, daughter nuclei with cytoplasm released in the environment (water or Blood) etc.

Each daughter schizont or pseudopodia spores forming gradually into young.

BUDDING IN PROTOZOA:-

• Asexual reproduction by budding takes place in many protozoa.

E.g., – Class :- Suctoria: vorticella, Ephalota

           Class :- Flagellata: Acinata

• By budding, one or more smaller individuals are formed from single parent body. Budding may be Exogenous or Endogenous.

 

IN EXOGENOUS BUDDING:- 

Buds develops as an out growth on the parent body, Which after sometime detached and leads free life.

 

IN ENDOGENOUS BUDDING:-

The buds are produced inside the parent’s body in special space called brood pouch or brood chamber. After sometime, bud detached from the parent body and leads free life.

 

PLASMOTOMY IN PROTOZOA:-

In certain multinucleate protozoa like OPALINA and PELOMYXA, the cytoplasm divides but nuclei don’t divide. Each daughter cell regains the normal number of nuclei by nuclear division. 

   

 

 

 

CHARACTERS AND CLASSIFICATION OF PORIFERA

Robert E. Grant, used the term of porifera in 1839. Meaning of porifera is pore bearer. Therefore, those organism in which body bears pore are called porifera.

 

CHARACTERS:-

• Porifera are diploblastic i.e., the two layers are found in the body (Ectoderm and Endoderm).
• The animals of porifera are multicellular.
• All animals of porifera are marine found in sea water (except spongilla, fresh water).
• These are plant like fixed and generally cylindrical in shape.
• Over body surface, pore or ostia are found. From the ostia water enters into the body.
• Definite organs and systems are not found. It has cellular level of body organization. 
• Digestion intracellular.
• Water current enters in the body helps in feeding and respiration, reproduction and excretion.
• Nervous system absent.
• Reproduction takes place by asexual and sexual method.
• Asexually reproduction by budding. Sexual reproduction by gametes or sperms and ovum.

The whole body is formed of 3 types of spicules. Spicules helps as Skeleton. On the basis of spicules, Phylum porifera has been classified into 3 classes-

1. Calcarea or calcispongae
2. Hexactenillida or Hyalospongae
3. Desmospongae

Class- Calcarea

• In this class calcareous spicules are found.
• In this class small sponges are found. These sponges are found all over the world in sea water.
• Canal system is found either Ascon type or Sycon type.
• This class has been divided  into 2 orders-

1. Homocoela
2. Heterocoela

 

Order -  Homocoela

• The body is simple cylindrical.
• The inner most layer of body is lined with flagellated collar cells.
• Body wall thin.
• Canal system Ascon type.

Example – Lecucosolenia

• Spongocoel is lined by flagellated cells.

 Order- Heterocoela

• Spongocoel lined by flat cells.
• Flagellated cells found in flagellated chamber.
• Body wall thick and folded.
• Complicated type of canal system syconoid or lecuconoid type.

Example :- Sycon or Grantia

Class :- Hexactenillida

• In this class large size sponges are placed.
• These sponges are found depth of sea, nearly 300 ft below.
• Sponges are cylindrical, Cup shaped, funnel like, pitcher like, flower vase like etc.
•  Spicules are triaxon type.

• Body glass like beautiful. e.g., glass sponges (Hyalonemma)
• Canal system Rhagon type.
• This class has been divided into 2 orders-

1. Hexasterophora
2. Amphidiscophora

 

Order :- Hexasterophora:

• In the mesenchyme Hexasters spicules present.

• Body skeleton frame like, irregular.

e.g :- Euplectella  (Venous flower basket)

 

Order :- Amphidiscophora

• Hexasters absent, but amphidiscs monoxon spicules present.
• Both sides of spicules bear crown of rays.

• Sponges are spiral rope like. e.g :- Glass sponges.

 

Class :- Desmospongia

• Skeleton are absent in this class of sponges.
• In some sponges, Spongian fibres or siliceous spicules present.
• All are generally marine.
• Body rounded, cup shaped or club shaped.
• Canal system Leucon type.
• This class has been divided into 4 orders :-

1. Tetractenillida
2. Monoaxonida
3. Keratosa
4. Myxospongia

 

ORDER :- TETRACTENILLIDA

• Tetraxon spicules present, sometimes spicules absent.
• Sponges are round or flat. e.g :- Chondrilla

ORDER:- MONAXONDIA

• Monaxon Spicules present.
• Shape flask like, branched, found attached with  Mollusca shell. e.g :- Cliona, fresh water Spongilla.

ORDER :-  KERATOSA

• Spongilla spicules present.
• Body rounded leathery. e.g.:- Horse sponge.

ORDER :- MYXOSPONGIA

• Skeleton absent in this type of sponges.
• Body blistering. e.g :- Oscarella ( Blacksponges), Heliosarca   
•     

Helminths :- Type study- Fasciola hepatica or liver fluke

 

Habit and Habitat of liver fluke –

Living fluke or Fasciola hepatica is digenetic trematode which is found in the bile passage and liver of sheep and other domesticated animals. It complete its life cycle in snail (pila). It enjoys a worldwide distribution. As this animal completes its life cycle in two hosts, Hence it is called as digenetic.

Internal organization

• Body wall – Body wall of Fasciola or liver fluke is characteristics and adapted for parasitic of life. It has no cellular epidermis and consists of the following layer :-
• Outer layer is thick and homogenous, formed of cuticle. It is made up of scloroproteins and bears many spines over it.
• Below the cuticle a thin basement membrane is present.
• Below the basement membrane, a sub- cuticular muscle layer is present. It is of three layers :-

a.       Outer layer of circular muscles.

b.       Middle layer of longitudinal muscles.

c.       Inner layer is called diagonal muscle layer.

• Parenchyma or Mesenchyma :- The interior of the body, between the organs is occupied by large number of branching, fluid filled parenchymatous cells.
• Gland cells :-  Many unicellular gland cells are found embedded in the muscular layer is called as gland cells .

Digestive organs

In Fasciola the digestive system is complex and digestive organs are branched. It bears mouth but anus is absent.

Digestive organs consists of the following parts:-

a.       Mouth – It is a small opening, situated ventrally at the apex of head lobe. Mouth is surrounded by oral sucker.

b.       Oral chamber – Mouth opens behind into a short, funnel shaped oral chamber.

c.       Pharynx – Pharynx is short rounded and muscular thick wall tube. Oral chamber leads into pharynx. It bears pharyngeal glands.

d.       Esophagus – Esophagus is a short, narrow structure opens behind into the intestine. Pharynx opens into esophagus posteriorly.

e.       Intestine – Intestine of Fasciola is divisible into lateral - 2 branches. Each lateral branch is branched and running on either side of the body up to the posterior end. Posterior part of intestine is closed. A large number of caeca or diverticula an given out along either side of the whole length of the intestine. The diverticula of outer side is highly branched and large. But the diverticula of inner side are short and unbranched.

Excretory system

The excretory system of Fasciola is consists of-  

1. Branched excretory duct.
2. Gland cells
3. Excretory duct

• There is a control or main excretory canal found in the body of Fasciola. Excretory canal is running medially for about 3/4^th of body length posteriorly. It opens outside through an excretory pore. Excretory pore is present mid ventrally at the posterior end of the body.
• From the main excretory -2 Antro-dorsal excretory ducts and 2- Antro- ventral duct arises. Antro-dorsal excretory ducts of Antro-ventral ducts gives off numerous branches on their sides. These side branches organ gives many minute branches is called capillaries. The end of minute capillaries forms flame cells.

The flame cells :-

• Each flame cells is present at the tip of the excretory duct. Each flame cell is an irregular mesenchyme cell with thin elastic cell wall, peripheral cytoplasm and a central intra cellular space.
• A bunch of long vibratile cilia flickers constantly like a flame inside the central space. By the flicking (vibration) of the cilia drives the parenchyma and pour into the excretory ducts. Waste products passes through the other excretory duct of reaches finally to the central excretory canal. As central excretory canal opens out side by excretory pore, waste materials are expelled out side the body of Fasciola.

Respiratory system

• The respiratory organs are absent because it selves inside the body, hence the respiration in anaerobic type (i.e respiration occurring in the absence of oxygen).
• It is completed by the fermentation of glycogen. Glycogen breaks into CO₂ and fatty acids of liberates energy.

Nervous system

Nervous system is well developed consists of :-

• A pair of cerebral ganglia.
• A nerve collar around the esophagus and connective the cerebral ganglia.
• Nerves from the cerebral ganglia to the head lobe and to the hinder part of the body.
• A pair of thick lateral nerves extending to the posterior end of the body and a pair of their dorsal and ventral nerves to the various organs of the viscera of the body.

Reproductive system

Fasciola hepatica are mostly hermaphrodite i.e; male and female reproductive organs are present in the same individual.

Male Reproductive organs :-

a.       Testes :- One pair of highly branched testes present. It is tubular. One testes lying behind other in the posterior middle part of body.

b.       A pair of vasa-deferentia:-  From each testes a narrow duct arises is called vasa-deferentia. The two vasa-deferentia runs forward and meet to form a common sperm duct and lying below ventral sucker.

c.       Seminal vesicle:- It is a pear shaped structure lying inside the cirrus sac. The common sperm duct dilates to form seminal vesicle.

d.       Ejaculatory duct:- It is a fine convoluted tube arises from the seminal vesicle. Ejaculatory duct runs forward through the cirrus and open into genital chamber through male genital pore.

e.       Cirrus and cirrus sac:- Cirrus or penis is a muscular structure. It can everted and drawn in through the genital pore. It helps in copulation.

The cirrus and the seminal vesicle both are enclosed in a bag like cirrus sac.

f.        Prostate gland:- Numerous unicellular prostrate glands are found around the ejaculatory duct.

Female Reproductive Organs :-

The following organs are included under female reproductive system :-

• .       Ovary :- There are single ovary in Fasciola. Ovary is large, tubules and highly branched. It is situated on the right side in front of testes.
• .       Oviduct :- Oviduct is short . It arises from the inner side of ovary. It runs down wards and back wards, to join uterus. During this course it gives laucers canal.
• .       Vitelline gland and vitelline duct:- Numerous minute vesicles like structures are found on lateral side of the body. These are called as vitelline glands. A fine duct arises from each vesicle and after uniting with similar ducts from other vesicles opens into the lateral longitudinal duct. The two longitudinal ducts are connected in the middle line by transverse vitelline duct. A median duct arises from the transverse vitelline duct and joins the oviduct.
• .       Laucers canal :- Laucers canal arises from the oviduct and acts as sperm duct. It opens outside during breading seasons.
• .       Ootype :- The place of union of oviduct and vitelline duct becomes swollen. It is called ootype. Inside the ootype eggs are collected.
• .       Shell gland or Mehlis glands:- Many unicellular glands are surrounding the ootype is called shell gland. The Secretion of shell glands secretes the shell around the eggs. The secretion makes the passage of uterus smooth.
• .       Uterus :- Uterus arises from ootype as a wide tube. Uterus runs forwards and opens into genital atrium. It opens on the left side as female genital aperture. It is filled with fertilized eggs. The terminal part of the uterus are muscular.   

• 

  Reproductive System of Fasciola hepatica 

ANNELIDA- EXCERETORY SYSTEM OF LEECH

The excretory system of leech can be studied under the two headings-

1. Excretory organ
2. Process of excretion

Excretory organ-

Nephridia are the main excretory organ of leech. Nephridia are segmentally arranged from 6^th to 23^rd segment. Each has hollow, microscopic tubes arranged in pairs. There are 17 pairs nephridia are found in Leech.

These are of two types :-

1.  Pretesticular Nephridia - 6 pairs
2.  Testicular Nephridia- 11 Pairs

Pretesticular Nephridia :-

 This type of Nephridia are found from 6^th to 11^th segment. This type of nephridia has no connection with testis; hence called as pretesticular nephridia. Haemocoelomic ampullae and ciliated organs are absent in this type of nephridia. In other structures they are similar with testicular nephridia.

 Testicular Nephridia :-

Testicular Nephridia are found from 12^th to 22^nd segments. They are attached with testis sacs. This is called typical nephridia and 11 pairs in number. Each testicular nephridium is horse shoe shaped in outline. It consists of six parts-  

     1.Main lobe   2.Vesicle and vesicle duct   3.Apical lobe  4.Inner lobe   5.Initial lobe   6.Ciliated organ

Main lobe:-

It is exactly horse shoe proper. Main lobe of the nephridium lies between the caeca of two crops. It is the main parts of nephridium and divisible into.

• Long anterior part - Anterior limb
• Short posterior part - Posterior limb

The cells of main lobe, consists of glandular polyhedral cells. The cells are connected with small canaliculi. Canaliculi of main lobe opens into central canal.

Anterior limb opens into vesicle duct and posterior limb connected with apical lobe.

Vesicle and vesicle duct –

The anterior limb of the main lobe continuous to form a duct called vesicle duct. Vesicle duct bends posteriorly and opens into vesicle.

Vesicle is a large, oval, non-contractile, sac like structure. The wall is thin and inner lining is ciliated.

 A small thin tube arises from vesicle and opens outside ventrally through nephridiopore. Sphincter muscles are found around nephridiopore.

 Apical lobe –

Posterior limb of the main lobe proceeds forward and forms apical lobe. It is found below the crop. Apical lobe is thick, stout and bends. It appears like the head of walking stick. The cells of apical lobes are larger. Intracellular canals are found in it. This canal opens into central canal.

 Inner lobe –

 It is a short lobe called as incurrent lobe. It is found between anterior and posterior limb. Inner lobe is continued to the outer border of apical lobe up to middle. It joins with main lobe in posterior side.

Initial lobe –

It is associated with apical lobe. It is thin long transparent cord like structure. It surrounds the apical lobe. Initial lobe is formed of single row of hollow cells. The anterior end of this lobe is closed but the posterior end opens into main lobe. Anterior end is situated near perinephrostomial ampulla on testis.

Intra cellular canal is present in the initial lobe. Intra cellular canal gives rise to diverticula in each cell.

Ciliated organ –

Ciliated organs are peculiar structure, present in perinephrostomial ampulla. Ciliated organs are suspended from the wall of ampulla by 4-5 strands or trabeculae.

Ciliated organs are similar to the nephrostome of the typical nephridium. Ciliated organ is a modified compound structure, divides into two parts-  

1. Central reservoir – Spongy
2. Ciliated funnel

Reservoir- It is formed of connective tissue spongy central mass. It forms coelomic corpuscles. The wall of this is unicellular a perforated. This aperture opens into ciliated funnel.

Ciliated funnel – Each ciliated funnel is small earlobe like. Th margins are ciliated.

In embryonic stage, ciliated organs are attached with nephridium but in adult Hirudinaria (Leech), it looses its connection. Ciliated organs are non-functional in adult. It becomes a part of haemocoelomic system and forms coelomic corpuscles.

The different lobes of Nephridium consists of gland cells. It bears intracellular canals or ducts inside. The ducts of different lobes opens into wide central canal. Central canal opens into vesicle after 1 and 1/4 turn.

Process of excretion 0r Physiology of excretion - 

• The ciliated organ is completely separated from the nephridium in adult stage. Ciliated organ has no excretory function in adult but it manufacture coelomic corpuscles of haemocoelomic system.
• The nephridium proper is excretory in function. It serves to eliminate water and nitrogenous wastes from the body. Nitrogenous consists of mainly ammonia. Quantity of urea hence leech is a Ammonotelic.
• Nephridia acts as excretory as well as osmoregulatory organ. Each nephridium is richly supplied with branches of haemocoelomic channels. The gland cells absorb excretory products from the haemocoelomic fluids. Excretory fluid is finally collected in the vesicle of nephridium and lastly expelled out through nephridiopore.
• Nephridia are osmoregulatory in function because maintained water balance in the body. The osmotic pressure of body fluid is always higher than the surrounding water. Hence water enters into the body continuously.
• The Nephridium removes water from the body. Leech has special mechanism in the epidermis, to absorb Na + Cl ions to opens at the loss in metabolism.                                                                      

COELENTERATA- LIFE CYCLE OF OBELIA

 

• In obelia colony, reproduction takes place by asexual methods and produces polyp or hydranth and blastostyle.

1. Polyp or hydranth
2. Blastostyle

• Both are asexual zooids. These two zooids represent asexually generation. Blastostyle, produces medusa. Medusa is reproductive zooids. In medusa four gonads are present.
• In medusa, gonads are either male or female. Therefore, medusa is unisexual.

Gonads

In medusa, gonads are found in sub-umbrellar surface. Gonads are per radial in position. Each gonad is round and formed of interstitial cells. Interstitial cells are present between ectoderm and endoderm.

In male medusa, interstitial cells forming, spermatozoa but in female medusa interstitial cells produces ova.

After the maturity of female or male gonad, ectoderm raptures, and spermatozoa or ova releases in water and swims freely in sea water.

Fertilization

The sperm of one medusa swims in water and reaches to the ova of other medusa. Sperms and ova together fuses. In this way cross fertilization takes place.

Generally, sperms reach to ovum by water current. Sperm is male gamete and ovum is female gamete. Male gamete and female gamete unite to form single zygote.

Development

Zygote divides holoblastic manner. Division of zygote is called cleavage.

Cleavage -  

Zygote first divide to form four celled stage.

First and second cleavage is vertical.

Third cleavage is horizontal, forming eight celled stage.

Then, divides and redivides to form hollow structure called Blastula stage.

 The cavity of Blastula is called Blastocoel.

 The wall of Blastula is one cell thick.

Stereo gastrula 

The cells of Blastula divide and redivide to fill up the Blastocoel cavity. It is called solid gastrula. It is also called stereo gastrula. The outer wall of gastrula is called ectoderm and the inner wall of gastrula is called endoderm. After sometime it elongates and ectodermal cells develops cilia. Gradually in the center a cavity is formed. It is called coelenteron. Now, gastrula changes into planula larva. Planula larva swims in water freely.

Hydrula 

After some times the free swimming planula larva fixes with the free swimming planula larva fixes with any substratum (stone or wooden logs). Now changes took place in planula. The anterior part of planula forms basal disc, but the posterior part of planula forms manubrium. The terminal part of manubrium breaks to form mouth. Many tentacles develop around mouth. This stage is called hydrula. The basal part of hydrula, is called Hydrorhiza. Hydrorhiza grows on substratum and gives vertical branches.  vertical branches grow upward. From the vertical branches buds arises. Due to budding polyp and blastostyle are formed. Gradually, a complex obelia colony is formed. Blastostyle, slowly and slowly form medusa buds and starts sexual generation. Medusa is swimming stage in obelia. It works for dispersal. It is essential for survival.

Alternation of generation or metagenesis

Generally, in the life cycle of particular organism, after sexual generation, asexual generation is found. The beginning of sexual generation starts from sex cells. Sex cells are formed in parent body but asexual generation are formed after division of sexual generation.

The individual of asexual generation don’t bear gonads.

Sexual generation are formed by division of asexual generation. In sexual generation, gametes (female & male) are formed.

 This process in which sexual generation and asexual generation is found alternately is called Alternation of Generation. Today, it is called metagenesis.

In the life cycle of obelia, it is a good example of alternation of generation. This process is going on continuously.

The hydra like colony of obelia is showing asexual generation. But medusa forms sexual generation. The hydroid colony of obelia bears no gametes or sex cells. The colony reproduces, asexually by budding, forming polyp and blastostyle. 

Blastostyle produces medusa. Medusa by sexual reproduction produces gametes. Formation of gametes takes place by gametogenesis.

After the fusion of male and female gametes, zygotes are formed. In this way regularly metagenesis occurs. Although, in obelia true alternation of generation is not found.

Because, hereafter diploid generation, haploid generation is not found. In obelia only gametes are haploid and sexual medusa bears gonads. Gonads are diploid like hydroid form.

Conclusion

In this way medusa is sexual form. It bears sex organs and swims freely. It produces male or female gametes. Sexual and asexual generation is clearly seen in obelia.   

 

 

 

Porifera: Canal System in Sponges

 

• The arrangement of passage way in the leading from incurrent dermal pores to the spongocoel then out through the osculum is known as canal system.
• There are three types of canal system are found in sponges-

1. Ascon type- Leucosolenia
2. Sycon type- Scypha
3. Leucon type- Spongilla 

Ascon type-

• This type of canal system is found in simple sponge like- Leucosolenia. The body is simple vas like. Dermal pores are found on the body surface. Incurrent pores are intracellular pores leading through tubular porocyte. Porocyte opens directly into the spongocoel. The entire spongocoel lined by flagellated cells or choanocytes. Spongocoel opens at the tip of sponge is called osculum.
• Water current from the outer side enters through ostia or dermal pores into the spongocoel. Lastly water goes out through osculum.

             Water from exterior--- Ostia --- Spongocoel ---Osculum --- Exterior 

Histology

Body wall consists of outer layer of flattened cells called pinacocytes. Inner layer is of flagellated cells. Between outer layer (ectoderm) and inner layer (endoderm), mesenchyme present. Mesenchyme contains- Amoebocytes, Archeocytes and skeletal spicules.

 

 

 

 Sycon type-

• This type of canal system is found in sycon or schypha. Sycon type is advanced over ascon type of canal system. Sycon type of canal system has been evolved from ascon type of canal system.
• The body of sycon is thick and massive. Thick body wall is due to the folding of simple ascon type. Due to the out pushing of the body wall radial canals or flagellated chambers are formed. These out pushing are at regular intervals. Between two out pushing incurrent canals are present. Incurrent canals are lined by flat ectodermal cells or pinacocytes. Flagellated chambers or radial canals are lined by choanocytes cells.
• Between the radial canals and incurrent canals, small pores are present called prosopyle. Prosopyle is intracellular with porocyte.
• Incurrent canals opens outside on the surface of body by minute pores called ostia.
• The flagellated chambers opens towards paragastric cavity or spongocoel by an aperture is called apopyle. Apopyle opens into excurrent canal and excurrent canal opens to the spongocoel through gastric ostium.
• Spongocoel opens outside through terminal aperture called osculum.
• Incurrent canals lined by dermal flat pinacocytes. Flagellated chambers lined choanocytes. The spongocoel of sycon or scypha is lined by the pinacocytes cells.
• The course of water current is given below in this type of canal system.

Exterior water --- ostia --- incurrent canal --- prosopyle --- radial canal --- apopyle --- excurrent canal --- gastric cavity --- spongocoel --- osculum --- exterior water

In this way water current passes through a number of canal systems. Flagellated canals and incurrent canals are radially arranged around the spongocoel.

 

 

 

 

Leucon type

• This type of canal system is complex than the sycon type. In this type of canal system, single flagellated chamber or radial canals are subdivided into many small rounded or oral flagellated chambers. This is due to the out folding of single flagellated chambers of sycon type. Mesenchyme becomes thick, flagellated chambers irregularly arranged in incurrent canals. Excurrent canals develop between radial canals and spongocoel.
• The excurrent canals from different radial chambers opens into wider channel and finally opens to the outside through osculum. Similarly the incurrent canals also get branched and irregularly arranged. The dermal ostia may lead directly into the incurrent canals or opens into the subdermal spaces crossed by the spicules.
• Leucon type of canal system has three grades of canal systems-

1. Eurypylous canal system
2. Aphodal canal system
3. Diplodal canal system

Eurypylous canal system- 

This is the simplest type of leucon type of canal system. It bears the structure like above description. Following in the course of water current in eurypylous type.

Outside --- Dermal Ostia ---Subdermal space & Incurrent canal ---Prosopyles ---Radial canal --- Apopyles --- Excurrent canals --- Large channels --- Osculum --- Exterior

Aphodal canal system-

•  In certain leuconoid sponges such as Stelleta and Geodia has aphodal canal system is found. 
• In this case apopyles are not a small opening but it is a narrow canal. It is called as aphodus. Aphodus opens into excurrent canals. Excurrent canals opens into spongocoel of sponge then out through osculum.

Outside --- Dermal pores --- Incurrent canals --- Prosopyle --- Radial canals --- Aphodus --- Excurrent canals --- Spongocoel --- Osculum --- Exterior  

Diplodal canal system- 

This type of canal system is found in Spongilla and Oscarella. In this case, both apopyles and prosopyles are in the form of tubular canals. In this way incurrent canals opens into radial canals through prosodus. Radial canals opens into incurrent canals through aphodus.

Euryphalous and Aphodal respectively 

Diplodal 

Rhagon type

In non-calcareous desmospongia leuconoid type of canal system is derived from larval stage. This is called as Rhagon. The canal system of rhagon is called as rhagon type. It has flattened pyramid like form with broad base (hyposphere) and a spacious spongocoel. The upper lateral walls passing a row of small oral flagellated chambers. The dermal pores leading into the flagellated chambers called as prosopyles. The openings of flagellated chambers into the spongocoel is called as apopyles. The spongocoel opens to the exterior by osculum.