NERVOUS SYSTEM OF HERDMANIA

Written by Friday, 07 July 2017 18:29
Herdmania shows brain or nerve ganglion. It is 4 mm long. It is present in the mantle in between the two siphons. A neural gland is present above the brain. From the brain three nerves arise, one goes to the bronchial siphon, and two will go to the atrial siphon. The brain represents the degenerated nervous system of the larva.
 
Receptors:
 
  1. Red pigmented spots on the test are photoreceptors. They are sensitive to light.
  2. Sensory cells of the margins of siphons and tentacles are tango receptors. They are sensitive to touch.
  3. The cells on the margins of siphons are rheo receptors. They are sensitive to water currents.
  4. Cells lining the siphons are thermo receptors. They are sensitive to changes of temperature.

EXTERNAL CHARACTERS OF HERDMANIA

Written by Friday, 07 July 2017 18:09
Herdmania is a simple ascidian, In Indian ocean this genus is represented by 4 species.
 
1. Herdmania pallida 2. H. ennurensis 3. H. mauritiana 4. H. ceylonica
 
Herdmania belongs to,
 
  • Phylum: Chordata
  • Sub-phylum: Urochordata
  • Class: Ascidiacea
  • Order: Pleurogona
 
Herdmania is a marine and sedentary animal. It is fixed to rocky substratum by a flat base. When it is disturbed, it suddenly contracts its body, and emits inner contents with force through its apertures. Hence it is called Sea squirt.
 
External Features:
 
It is potato like in shape. It is pink in colour. On the free side, body shows two projections, the brançhial and atrial siphons. The branchial siphon is short. The branchial siphon shows a branchial aperture or the mouth. The atrial siphon is longer. It bears the atrial aperture. Both the openings are bounded by four lips.
 
Herdmania - External Characters:
 
Test of Herdmania:The body of this animal is covered and protected by test. It is a thick, leathery covering of the body. It is secreted by the epidermis of the body wall. It has matrix, corpuscles, fibrils, blood vessels and spicules.
 
The matrix is composed of tunicin, which is cellulose. The cells in the test are of six types, large eosinophilous cells, amoeboid cells, small eosinophilous cells, vacuolated cells, receptor cells and nerve cells.
 
Fine fibrils present in the matrix. In the test blood vessels are present. In the test the spicules are calcareous spicules. They are microscleres, and megascleres.
 
Herdmania test thumb13
 
The test protects the body. Anchors the animals to substratum. Its spicules form a supporting frame work.
 
Herdmania Body Wall:
 
The body-wall of Herdmania is called Mantle. It is thick, and muscular in the antero-dorsal region of the body. It is thin, non-muscular and transparent in the postero ventral region. It shows epidermis, mesenchyme, and inner epidermis.
 
1. Epidermis: It Is single layer of cells. It covers the bronchial and atrial apertures and siphons. The epidermis is interrupted at places where spicules and blood-vessels pass from the mantle into the test.
 
2. Mesenchyme: It develops from the mesoderm. It has connective tissue containing blood-sinuses, muscle-fibers, nerve fibers and cells. The muscle fibers are long and flat. They contain large nuclei.
 
Herdmania Body wall thumb23
 
3. Inner Epidermis: It is single layer of flat cells. It forms the lining of the atrial cavity.
 
  1. The body-wall protects visceral organs.
  2. The outer epidermis secretes the test.
  3. The musculature brings contraction of the body and the siphons.
 
Herdmania Atrium:
 
In Herdmania coelome is not developed. Atrium is a spacious ectoderm lined cavity. it is covered by the mantle A part of the atrium surrounds the pharynx. The stigmata of the pharynx open into this cavity. Part of the atrium is dorsal to the pharynx. It is very wide and is called cloaca. The rectum and gonoducts open into this. The cloaca opens to the exterior through atrial siphon and trial aperture. The atrial siphon shows a ring of processes called atrial tentacles at its base.

AFFINITIES OF UROCHORDATES

Written by Friday, 07 July 2017 17:08
The typical tunicate looks like a non-chordate animal. If the life history of such animal is studied, the larval form reveals the chordate characters of that animal.

In 1816, Lamarck and Cuvier placed these animals in one class ‘Tunicata’. Allis described a compound Ascidian “Botryllus’ in 1756. KowalevskSi. Worked on the development of the Ascidian and placd them in tunicates after observing the chordate features.
 
Urochordates Resemblance With Chordate:
 
  1. Presence of dorsal tubular nerve cord.
  2. Presence of Notochord.
  3. Well developed pharynx with gill-slits.
  4. Presence of endostyle on the ventral side of the pharynx.
  5. Presence of atrium around pharynx.
  6. Presence of post - anal tail, with tail fin.
 
Because of these chordate features tunicates are included in chordates group.

Urochordates Resemblance With Amphioxus:
 
  1. Presence of notochord.
  2. Presence of dorsal tubular nerve cord.
  3. Presence of large pharynx with gill slits.
  4. Presence of atrium and atriopore.
  5. Presence of muscle band.

Thus, the Urochordates show close relation with Cephalochordates.
 
Urochordates Differ With Other Chordates:
 
  1. Presence of retrogressive metamorphosis.
  2. Absence of segmentation.

Because of these characters zoologists included these animals in a separate sub-phylum Urochordates.

During recent years many zoologists regarded the tunicates as primitive and ancestral forms to chordates as a whole.

CLASSIFICATION OF UROCHORDATA

Written by Friday, 07 July 2017 16:29
Subphylum Urochordata is divided into three classes.
 
  • CLASS 1.  ASCIDIACEA
  • CLASS 2. THALIACEA
  • CLASS 3. LARVACEA (APPENDICULARIA)
 
CLASS 1. ASCIDIACEA:
 
  1. These are sedentary tunicates.
  2. The body is covered by a test.
  3. Pharynx is large and contains gill-slits.
  4. Notochord, nerve-cord and tail are absent
  5. These are Bisexual animals.
  6. Life-history includes a typicalTadpole larva. The class Ascidiacea is divided into two orders.

Order 1. Enterogona

These ascidians bear one gonad in the intestinal loop. Neural gland is ventral to the ganglion. Tadpole larva is seen.

Ex: Ascidia and Ciona.

Order 2. Pleurogona:
 
In these ascidians,gonads are paired and are present in the atrial wall. Neural gland is dorsal to the ganglion.
 
Ex: Herdmania, Botryllus.
 
CLASS 2. THALIACEA:
 
  1. These Urochordates are free-swimming and pelagic forms.
  2. They are covered by transparent test.
  3. The brachial and atrial apertures are placed at anterior and posterior ends.
  4. Pharynx is small.
  5. Gill-slits number is less.
  6. Notochord, nerve-cord and tail are absent in the adult.
  7. Asexual reproduction is by budding.
  8. These are bisexual animals.
  9. Tailed larva may be present or absent.
  10. Alternation of generations can be seen in the life history.

The class thaliacea is divided into three orders.

1. Doliolida, 2. Pyrosomida and 3. Salpida.

Order 1. Doliolida (Cyclomyarla):
 
  1. Barrel shaped body is completely covered by Muscle bands.
  2. Pharynx is small.
  3. Number of gill slits will be small.
  4. Tailed larva is seen.
  5. Sexual Blasto-zooid and asexual oozooid stages will alternate in the life cycle.
 
Ex: Doliolum.

Order 2. Pyrosomlda:
 
  1. This order includes colonial forms.
  2. Muscle bands are small and present at the ends.
  3. Gill-slits are many.
  4. Tailed larval stage is absent.

Ex: Pyrosoma (Luminescent colonial form).

Order 3. Salplda (Hemimyaria):
 
  1. This order includes organisms whose body is prism like.
  2. Muscle bands are complete dorsally and incomplete ventrally.
  3. Only one pair of lateral gill slits are present.
  4. Tailed larval stage is absent.
  5. Life history includes alternation of generations.
 
Ex: Salpa.

CLASS 3. LARVACEA (APPENDICULARIA):
 
  1. These are free - swimming, pelagic tunicates.
  2. True test covering is lacking.
  3. They show loose gelatinous house.
  4. This house is useful for filter feeding.
  5. Two gill slits re present.
  6. Atrium is absent.
  7. Notochord and nerve cord are Persistent.
  8. They show tail throughout their life.
  9. Neotenic forms are included.

Ex: Oikopleura.

GENERAL CHARACTERS OF UROCHORDATA

Written by Thursday, 06 July 2017 23:42
The tunicates were first regarded as sponges. Lamark in 1816 placed Tunicata in between the Radiata and Vermes in his system of classification. Later, they were included in Mollusca. In 1866 Kowalevsky kept them in chordates.
 
Their chordate features are clearly seen in the larval stages. All Urochordates are marine and occur in all the seas. Majority of them are sedentary and some are pelagic.
 
  1. Body shows variation in size and form.
  2. The body is un segmented and has no tail.
  3. The body is covered by a test. It is formed by tunicine which is rallied to cellulose. Hence the name Tunicata.
  4. Body wall shows one-layered epidermis, dermis is made by connective tissue and muscles, and atrial epithehum.
  5. Celome is absent.
  6. Atrial cavity surrounds the pharynx, into this cavity the gill slits,anus and genital ducts will open. It opens through atrial aperture.
  7. Larva has notochord in the tail. It disappears during metamorphosis.
  8. Respiratory system contains gills in the pharyngeal wall.
  9. Ciliary mode of feeding is common.
  10. Open type of Circulatory system is seen.
  11. The heart is ventral and it periodically reverses its function.
  12. Nervous system is represented by a single dorsal ganglion in the adult.
  13. Excretion is carried on by nephrocytes.
  14. Asexual reproduction is by budding.
  15. Bisexual animal and cross fertilisation is favored.
  16. Fertilization is external.
  17. Development includes a minute, free swimming tadpole larva with a tail, a dorsal nerve cord, and a notochord in the tail. In some urochordates retrogressive metamorphosis is seen in the life history.

ORGANOGENY IN AMPHIOXUS

Written by Thursday, 06 July 2017 23:11
The development of organ system is called as Organogeny. In Amphioxus the gastrula elongates in the future anteroposterior direction and the blastopore comes nearer to the posterior end of the dorsal surface and starts the development of organ system.
 
1. FORMATION OF NEURAL TUBE INAMPHIOXUS: Neural ectoderm cells of the middorsal line became thickened and elongate. It forms a flattend neuralplate. The neural plate sinks and the ectoderm at the sides rises up to form neural folds. This is from the blastoporal end and extend forward. The neural folds are fused and form the neural tube. It opens at the anterior end by a small opening called neuropore.
 
The cavity of neural tube gets separated from the cavity of archenteron. The neural tube forms the nerve cord of the adult.
 
2. FORMATION OF NOTOCHORD IN AMPHIOXUS: Along with the development of neural tube the Notochord development starts. The presumptive Notochordal cells are present mid dorsally in the roof of archenteron and below the neural tube. They evaginate from the endoderm and constrict off to form solid mass of cells. This is started at the anterior end and continues back- ward towards the hind end of elongated gastrula. This row of cells is known as notochórd. The notochordal sheath will develop around the notochord from the sclerotome of mesoderm and development is completed.
 
3. DEVELOPMENT OF MESODERMI AND COELOM IN AMPHIOXUS: The presumptive mesoderm is present in the dorso-lateral walls of the archenteron. It surrounds the notochord on both the sides. From the wall of archenteron two folds arise in the form of grooves. They divided into a series of segments by several transverse lInes. Each segment is called coelomic pouch. These coelomic pouches separate from the archenteron and form a series of coelomic sacs in between the ectoderm and endoderm. The (vail of the sacs forms the mesoderm. The coelomic sacs will grow in size and extend both up wards and down words and occupy the space between ectoderm and endoderm.
 
Each coelomic sac divides into two parts.
 
1. The dorsal part is the myotome, and 2. The ventral part is splanchnotome.
 
1. Myoteme: It contains a median muscular splanchnic portion and a laterally placed thin walled parietal portion. They surround the coelomic space or myocoel. The muscular part give dermatome which forms a thin sheet below the ectoderm of the skin.
 
Scierotome gives rise to a sheath of notochord, and nerve cord. Muscle plate enlarges and forms the muscle bundles.
 
2. Splanchnotome : It is formed of two sheets separated by space, called splanchnocoel. It forms the coelome of the embryo.
 
The outer wall of splanchnocoel present in contact with the ectoderm is called somatic layer of mesoderm. The inner wall of splanchnocoel which is in contact with. The endoderm of archenteron is known as splanchnic mesoderm.
The origin of coelom in Amphioxus is called enterocoelic type,brcause it is derived from the archenteron.
 
HATCHING IN AMPHIOXUS:
 
About 12 hours after fertilization the embryo hatches out It is elongated and laterally compressed. It is called larva.
 
The young larva of Amphioxus is without mouth and anus. It swims actively by means of cilia present on the epidermis. The larva gradually elongates. The notochord grows into the anterior end. The anterior end is pointed. The posterior end develops a caudal fin.
FATEMAP OF FERTILISED EGG OF AMPHIOXUS: Conklin in 1932 studied the fate map of Amphioxus. In 1962 Tunguntung described the egg of amphioxus 4 regions are clearly seen.
 
  1. Clear cytoplasm occupying the animal pole will mainly become epiderm& ectoderm.
  2. The granular yolky cytoplasm near the vegetal pole will become endoderm.
  3. The crescent shaped cytoplasm of the posterior side will become mesoderm.
  4. Opposite to this mesodermal crescent another crescent is present which becomes presumptive nervous system. The lower part will develop into notochord.
 
First Cleavage: After 1 1/2 hours* of fertilisation   meridional cleavage will take place. It is from anterior end to posterior end (The median axis of the egg is from Animal pole to vegetal pole). The result of this cleavageis two equal blastomeres are formed.
 
Second Cleavage: It is also meridional but it is at right angles to thç first one. Because of this cleavage 4 equal blastomeres are formed.
 
Third & Fourth Cleavage: It is in double plane. Both are, in meridional way extending from animal pole to vegetal pole. 8 micromeres and 8 macromeres are resulted.
 
Fifth Cleavage: It is latitudinal. It is in double plane. 32 celled stage is reached.
 
Sixth Cleavage: These divisions are approximately meridional. 64 blastomere are formed.
 
After the cleavage it is difficult to follow the cleavage pattern.
 
Cleavage: The fertilised egg will become a multicellular structure only because of cleavage. The cleavage is initiated by the holoblastic division and approximately equal further.
 
BLASTULATION IN AMPHIOXUS: From the 7th cleavage onwards irregular cleavages will occur. The cells of the vegetal pole will divide slowly where as the cells of the animal pole will divide in a quick way. Because of this blastomeres of the animal pole will be small and blastomeres towards vegetal pole will be bigger. The cells are loosely packed. It is called Morula. Then in the centre of the cell mess a jelly substance appear. It will push the blastomeres outside.Hence a single layered blastula is formed. It includes a fluid filled cavity celled blastocoel. This is called blastula stage. The blastocoel appears at the 4 celled stage as the cells are round and then do not unite.
 
GASTRULATION IN AMPHIOXUS: In amphioxus blastula is converted into gastrula by the following ways.
 
  1. Invagination
  2. Involution
  3. Epiboly
 
As gastrulation begins the mitotic activity in the primitive ectoderm cells will increase. Where as the endodermal blastomeres will not show mitotic activity. Because of this the endodermal cells of the vegetal pole will show invagination. In the beginning of the gastrulation the endodermal cells become plate like . This will show an in yard or invagination, in the blastocoel. It gives way for the development of archénteron. When the invagination is carried on the blastocoel becomes oblifrated. The embryo becomes cup shaped structures with two layer of cells. The opening is called blastopore. On the dorsal lip of the archenteron notochordal cells CARE are present.
 
amphioxus gatrula thumb10
 
INVOLUTION IN AMPHIOXUS: The notochordal cells should roll to the middle of the root of the archenteron. In the same way the mesodermal cells present on the ventral lip of the blastopore should rollinside. Themovement of ce& inside is called involution.
 
EPIBOLY IN AMPHIOXUS: As the changes are taking place, the ectodermal cells of the animaI pole will divide rapidly and they grow over the blastopore. This process is called epiboly. When the mesodermal crescent cells and notochordal crescent cells are totally shifted inside the lips of the biastopore will contract and the blastopore becomes very small. The two layered gastrula is formed. The outer ectoderm and inner endoderm with mesodermal notochordal crescent (cells) and central archenteron.

REPRODUCTION IN AMPHIOXUS

Written by Thursday, 06 July 2017 21:35
Amphioxus is a Cephalochordate animal. Its Life history in the early stages shows resemblance with ascidians. HATSCHE’K WILSON. CONKLIN’ worked on the part of embryogenesis of Amphioxus. ‘CONKLINS’ work in more accurate and is recent.
 
Amphioxus is a unisexual animal. But sexual dimorphism is absent. Amphioxus shows 26 pairs of Gonads. They are present from 25th myotomal segments to 51. These Gonads have no ducts. When mature the overlapping tissue of the Gonad will rupture and the gametes are Iibereted into atrium of Amphioxus. They will come out into the water through the atriopore.
 
AMPHIOXUS - ORGANISATION OF THE GAMETES
 
SPERMATOZOAN IN AMPHIOXUS: The mature male sex cell called Sermatozoan. It is 15 to 20 in length. It shows three regions.

1. Head, 2. Middle piece and 3. Tail .
 
On the head acrosome is present. Head shows a big nucleus. Around the nucleus thin sheet of Cytoplasm is present. It is called Manchetty. The middle piece is small with mitochondrial matrix called nebenkeron. The tail is long and shows movements.
 
EGG OR OVUM IN AMPHIOXUS: The mature female sex cell is ovum. It is small and 0.12 mm in diameter. It is a microlecithal egg. The cytoplasm around the nucleus will show yolk. In the peripheral cytoplasm yolk is absent. It is granular and is called carticoplasm. The plasma membrane surrounds the cytoplasm. Around this is a mucopolysaccharide membrane is present. It is called vitelline membrance. In between these two layers perivitelline space is present. The nucleus is present towards the animal pole, where as the opposite pole is called vegetal pole. The vegetal pole becomes posterio dorsal side of the embryo. The Animal pole becomes antero-ventral side of the embryo. Hence a gradient polarity is established in the egg.
 
FERTILISATION IN AMPHIOXUS: As soon as the egg comes in contact with water the vitelline membrane wilI separate from the plasma membrane.
 
The egg is surrounded by a number of sperms. One sperm will make its entry through the contents of the egg from the vegetal pole. At this time a number of changes take place in the corticoplasm. Now membranes are formed which unite with vitelline membrane It is called Fertilisation
 
So that no other sperm can enter into the egg. The head and middle piece of the sperm will enter into ooplasm of the egg. The egg nucleus undergoes second maturation division. Second polar body is pushed into the pervitilline space . The sperm nucleus and middle piece will show 180° twist and move towards the egg nucleus. Both the nuclei will unite. Thus a zygote nucleus is formed.

ATRIUM IN AMPHIOXUS

Written by Thursday, 06 July 2017 21:20
In amphioxus a big space is present, called Atrium it is lined by ectoderm. It encloses pharynx, and part of intestine. It opens out through atriopore at 36 myotome in front of caudal fin. The dorsal wall of the atrium is folded. ln the larva the atrium develops at the cost of longitudinal ridges called Metapleural folds on the ventral side of the body. They extend forward. The ridges of both the sides unite and atrium is developed.
 
In the adult the atrium is well developed. The pharynx opens into the atrium through gill slits. Water from pharynx enters into the atrium through gill slits. During this course respiration will take place.
 
This water will go out through atriopore.
 
The gametes are liberated into atrium. They are also, sent out through atriopore.

ENDOSTYLE IN AMPHIOXUS

Written by Thursday, 06 July 2017 21:07
In the Urochordate and Cephalochordate animals the ventral side of the pharyhx wilI contains endostyle. It is a glandular ciliated groove present on the floor of the pharynx. This contains alternate bands of ciliated and glandular cells. The gland cell secretes mucus. This mucus will concentrate the food particles of the water. By the ciliary action they are pushed forward in the endostylar groove. This concentrated food mass will enter into epipharyngeal groove through peripharyngeal grooves. Through epipharyngeal groove the food mass enters into the oesophagus.
 
Endostyle helps in the ciliary mode of feeding in, lower chordates.
 
In Amphioxus this endostyle is supported by a pair of skeletal plates.
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