Cephalochordata Classification systematic position:

Classification and systematic position of Cephalochordate :

The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single family, Branchiostomidae.The family contains only two genera Branchiostoma and Asymmetron.

The subphylum - Cephalochordata includes Amphioxus or Branchiostoma.

1., Pallas described this animal first. He named it as Umax lancealatus, He included this in Mollusca.

2. In 1834, costa described its chordata features.

3. In 1836, Yarrel named it as Amphioxus.

CHORDATE FEATURES OF CEPHALOCHORDATA (AMPHIOXUS):

1. Presence of dorsal tubural nerve cord.

2. Presence of a long notochord from anterior end to posterior end on the dorsal side. Because, it extends to the cephalic region. Hence it is called Cphalochordata.

3. Gill silts are present in the

4. Presence of post anal tail.

5. The presence of liver diverticulum.

6. The development of hepatic portal system.

7. Presence of Myotomes and which are useful for locomotion.

8. The presence of dorsal, caudal and ventral fins.

In these characters Cephalochordata resembles chordates.

CEPHALOCHORDATA PRIMITIVE CHARACTERS:

1. The excretory system contains protonephrldia.

In the chordate world, the presence of solenocytes is not report-ed. But, in Amphioxus solenocytes are associated with Nephridium)’

2. Absence of heart and kidney.

3. Absence of paired limbs or paired fins.

4. Absence of distinct head.

5. Absence of distinct paired sense organs.

6. Gonads are without gonoducts.

Thus cephalochordates will show many primitive characters

SPECIAL CHARACTERS OFCEPHALOCHORDATA:

1. Because of its ciliary mode of feeding, the pharynx is elaborated with many gill slits. .

2. Oral hood is well developed for ciliary mode of feeding.

3. Because of its ciliary mode of feeding the atrium is very well developed.

Thus, Amphioxus shows some special characters which are developed because of its ciliary mode feeding.

CEPHALOCHORDATES RELATIONSHIP WITH UROCHORDATES:

Cephalochordates show many close relationship with urochordata.

In some points they differ

Similarities:-

1. Presence of gill slits In pharynx.

2. Presence of Endostyle in Pharynx.

3. Presence of Ciliry mode of Feeding.

4. Presence of atrium.

Difference. :

1. Absence of test.

2. Absence of distinct heart

3. Presence of notoçhord and nervecord in the adults.

4 Presence of Myotomes in the adult.

In this way Cephalochordates differ with Urochordates.

Cephalochordates show some invertebrate features,

They are,

1. Presence of paired nephridia like annelids.

2. Presence of flame cells like Helminths.

3. Presence of soft body and slug like appearance like molluscans.

By considering the above facts we come to a conclusion that

Amphioxus is a chordate animal. Amphioxus is a degenerate jawless chordate animal. It mainly shows chordate features. It differs from urochordates in some aspects. Hence, It is separated and kept in a separate sub phylum called Cephalochordate.

Cephalochordata-General Characters

General Characters-Cephalochordata

Cephalochordata includes two genera, 1. Asymmetron and 2. Branchiostoma (Amphioxus). Cephalochordates are small fish like animals which show Chordate characters. The notochord extends the entire length of the body. They show a dorsal, tubular neural tube without a definite brain.

General Characters:

1. Body is fish -like and is useful for burrowing and swimming.

2. It has a head.

3. It shows a tail.

4. Appendages are absent.

5. Dorsal, caudal and ventral fins are present.

6. Body- wall shows one- cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle and parietal peritoneum.

7. It has no exoskeleton.

8. Notochord extends from the anterior end to posterior end.

9. Enterocoelic coelom is present. However, reduced in the pharyngeal region by atrium.

10. Alimentary canal is long. It includes a large pharynx with many gill-slits  ciliary mode of feeding is developed.

1 1. Gillss will perform respiration.

12. Circulatory system is closed.

13. Heart and respiratory pigments are absent.

14. Hepatic portal system is present.

15. Excretory system shows paired protonephridia with solenocytes.

16. Brain is not present

17. Two pairs of cerebral and several pairs of spinal nerves are present.

18. Sexes are separate. Gonads are metamerically arranged and with out gonoducts.

19. Asexual reproduction will not take place.

20. Fertilization is external.

Cephalochordata Classification:

The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single family, Branchiostomidae.The family contains only two genera Branchiostoma and Asymmetron

Amphioxus -Notochord:

Notochord in Amphioxus

In chordate animals notochord, dorsal tubular nerve cord and pharyngeal slits are present

In Amphioxus notochord is a cylindrical rod extending the whole length of the body from anterior end to posterior end. it is present below the dorsal nerve cord and above the alimentary canal. The notochord is formed b vacuolated cells. They are filled with fluid. These turgid cells will give strength to the notochord. This notochord is covered by a sheath of connective tissue.

In Amphioxus the notochord extends from anterior end to posterior end, hence it is placed in Sub phylum Cephalochordate. This notochord gives strength to the animal.Notochord is acts as a

skeleton of amphioxus

details about  Endostyle of Amphioxus

Amphioxus-Endostyle

Endostyle in Amphioxus

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.

Amphioxus-Atrium

Atrium in Amphioxus

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

AMPHIOXUS-ORGANOGENY

ORGANOGENY IN AMPHIOXUS.

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 IN AMPHIOXUS: 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.

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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.

AMPHIOXUS REPRODUCTION-POST FERTILISATIO CHANGES

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, and3) 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.

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.

previous topic discuss about Reproduction in Amphioxus

AMPHIOXUS-REPRODUCTION

REPRODUCTION IN AMPHIOXUS

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:

a) SPERMATOZOAN IN AMPHIOXUS: The mature male sex cell called Sermatozoan .

b) 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.

B, 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.

continue reading post fertilisation changes in egg of Amphioxus

AMPHIOXUS-SKELETON :

SKELETON IN AMPHIOXUS

Exoskeleton is absent in Branchiostama. Endoskeleton include the notochord, gelatinous rods and plates, fin-ray boxes2 etc.

1. Notochord in Amphioxus : The notochord of Amphioxus is extending the whole length of the body along the mid-dorsal line just below the central nervous system. The notochord consists of a single longitudinal row of cells. The row of cells is enclosed by a thin, elastic membrane surrounded by a thick layer of fibrous connective tissue called the Notochordal sheath.

Functions:

Its main function is to give support to the body and maintain its shape.

It permits slight bending movement of the body

2. Gelatinous Rods and Plates: The gelatinous rods give support to the bars of the pharynx, oral cirri and free anterior edge of the oral-hood. The rods and plates are formed by elastic fibers.

3. Fin-ray Boxes: They give support to the dorsal and ventral fins. They contain gelatinous matter and thick connective tissue covering.

AMPHIOXUS EXTERNAL CHARACTERS:

 EXTERNAL CHARACTERS OF AMPHIOXUS:
Amphioxus  belongs to Cephalochordate. In 1778 Amphioxus was first discovered and described by Palias . He called it a Molluscan animal. In 1834, it was described as a chordate animal by Costa. He called it ‘Branchistoma’. In 1836, Yarrell gave th name Amphioxus.
Amphioxus lives in shallow marine waters and burrows in sand. It is commonly seen in the Indian ocean. Most of its body is burned. Only its anterior part is left outside. It is active during night It is 3.5 to 6cms, in length lts body is whitish in colour. The body is laterally compressed. It is pointed at both ends. This spindle shaped body is useful for burrowing and swimming. The anterior end of the animal will project anteriorly as snout. It is called rostrum. Below this o al hood is present. The body is divisible into trunk and posterior short tail. Head is absent. Mouth is a wide opening present at the anterior end of the oral hood. It bears 10 to 11 pairs of buccal cirri. They test the purity of water. The oral hood encloses a cup like vestibule.,
The first two thirds of the body is triangular in section. The last on third body is nearly oval in section
AMPHIOXUS- FINS :
On the dorsal side dorsal fin is ‘present. It is connected with a caudal fin. Caudal fin is connected with ventral fin which extends up to atriopore.
The dorsal and ventral fins are supported by small rectangular fin- ray boxes There is a single row of Fin ray box in the dorsal fin. Two rows are present in the ventral fin.
Ventral surface of the anterior two-thirds of the trunk is nearly flat and its lateral margins are produced into a pair of thin folds, the meta pleural folds or lateral fins.
AMPHIOXUS- BODY WALL:The body is covered by skin.It has two regions, a-Epidermis. b. Dermis.
a) Epidermis : The epidermis is very thin layer of the cells. it is single layered. The epidermal cells are columnar and present on a basement membrane. They are ciliated or non ciliated. it has an outer cuticle covering.
The cuticle is made of a substance resembling chitin. The epidermis contains mucous cells and sensory cells.
b) Dermis : It is composed of connective tissue. it shows an outer thin layer with fibres and an inner thick spongy layer with connective tissue, blood vessels and nerve fibres.
Myotomes : The myotomes are thick and are in the dorsal and dorso-lateral regions. They are arranged in a linear series. They ‘are V shaped blocks. There are about 60 pairs of myotomes. The myotomes of the left side alternate with those of the right side. Each myotome is covered by connective tissue, called myocomtnata.
All the body muscles of Amphioxus are striated. These myotomes are helpful in swimming.

AMPHIOXUS- OPENINGS ON THE BODY:
. There are 3 openings on the body of Amphioxus.
1 . Mouth, 2. Atriopore and 3. Anus.
1 . Mouth : The mouth is very wide and lies at the anterior end of the trunk.
2. Atriopore : The atriopore is a small aperture situated in front of the ventral fin.
3. Anus The anus lies at the base of caudal fin on the ventral side.