TAXONOMY -NON CHORDATA AND CHORDATA

CHORDATES -GENERAL CHARACTERS The animal kingdom is divided into two sub-kingdoms:

1) Chordata and

2) Non-Chordata

CHARACTRS OF CHORDATES:

The Chordate characters appear in the developmental stage will persist in the adult or modified in the adult or disappeared in the adult

They are,

1. PRESENCE OF NOTOCHORD: It is an elastic, longitudinal stiff rod present between the nerve cord and alimentary canal. It is covered by an outer chodal sheath and inner elastic sheath or elastic-internal. Below it vacuolated, non-nucleated cells are present

Notochord is present in the embryos of the vertebrates, but is replaced by vertebral column. In less-developed vertebrates the notochord is present throughout the life.

2. PRESENCE OF PHARYNGEAL GILL SLITS : Gill slits are present on either side of the pharynx. Each gill slit develops in the embryonic stage by evaginations of endoderm in pharynx with a corresponding invagination of ectoderm on the outside of the body. They are useful for respiration. In reptiles, birds and mammals, there are several pairs of gil slits in embryonic life, but they are not functional hence they are closed.

3. PRESENCE OF DORSAL TUBULAR NERVE CORD : The nerve cord in chordates is a hollow tube, situated dorsal to the alimentary canal and the notochord. It develops from the ectoderm. The cavity of nerve cord is called neurocoel.

These chordate characters appear in the developmental stages and they may remain or change or disappear in the adult.

The following characters are seen in all most all chordates.

4. TRLPLOBLASTIC NATURE : They possess three germ layers,

1. Ectoderm, 2. Mesoderm, 3. Endoderm

5. DEVELOPMENT OF TRUE COELOME : In all the chordates a true coelome is present. It develops from mesoderm. The coelome is developed by enterocoelic method.

6. PRESENCE OF BILATERAL SYMMETRY : The body of the chordates show bilateral symmetry. Their body can be divided into two equal halves, through a sagittal plane which passé longitudinally.

No chordate animal possesses ideal bilateral symmetry, but they are near to such condition.

7. REDUCTION OF METAMERIC SEGMENTATION : Among the chordates, metamerism is seen in the internal structures. The myomeres are seen in lower chordates and segmentation is seen in the embryonic condition of higher vertebrates.

CHORDATES

The Phylum Chordate includes bilaterally symmetrical, metamerically segmented, triploblastic, enterocoelomate metazoans exhibiting hights complex organization. This phylum is supposed to be one of the most heterogenous and diversified gruop of animal kingdom. The minute sessile cephalodiscus, work-like Balanoglossus, degenerate tunicates, Amphioxus and the true vertebrates have been included in this phylum.

     The vertebrates are fishes, amphibians, reptiles, birds and mammals. All these animals appear to be quite different from each other .However all of them possess certain common characters. These common characters are the presence of Notochord, dorsal nerve cord and gill-slits.

Primary chordate characters:

1 . Presence of notochord or chorda dorsalis:

All Chordates porsess a solid, un segmented and flexible axial rod extending the whole length of the body. It is present mid dorsally and immediately above the alimentary canal and below the dorsal nerve cord. It porsess large vacuolated parenchymatous cells and enclosed in an inner elastic sheath and outer chordal sheath of dense fibrous connective tissue.

The notochord serves s a primitive internal skeleton. It also supports the central nervous system and the segmented muscles. In vertebrates the notochord is completely replaced by the vertebral column.

2. Presence of dorsal nerve cord:

The nervous system of chordates is the form of hollow tube present middorsally above the notochord and below the body wall. It is ectodermal in origin. From the ectoderm a medullary groove is formed which lateral develops neural tolds from its edges and forms the neural tube. Its cavity is known as Neurocoel. In higher chordates the anterior part of the neural tube develops into brain and the remaining part is called spinal cord.

3. Presence of Branchial or Pharyngeal clefts:

- In the life cycle of all chordates the branchial clefts are found universally at some stage. These are formed as perforations in the laterally of pharynx and open to the exterior. Such gill-clefts are developed in every chordate. In certain animals (Amphioxus) these remain throughout life. In Aquatic chordates (fishes) the visceral clefts develop vascular Lamellae gills. These clefts are known as gill-clefts or gill-slits.

In terrestrial chordates these are seen during early development but in the adults modified into lungs. In the lows chordates (hemichordate, Cephalochordate & fishes) visceral clefts help in feeding as well as assist in respiration. In certain terrestrial forms these re modified into endocrine glands.

Origin of Chordates:

There is no reliable evidence in favor of chordate origin. Origin of chordates has no definite conclusions because of lack of fossil evidences.

Recent investigations reveal the relationship among echinoderms, hemichordates and chordates. Certain phosphate is reported in all the three groups which is useful for muscle contraction Echinoderms and Hemichordates are developed from the common ancestor. So the chordates might have developed from the hemichordates.

The phylum chordata is classified into three sub-phyla. (Hemichordate has been given separate phylum status recently).

Simpler forms of chordate animals with notochord completely or incompletely formed are known as protochordates . These sub-phyla cephalochordata,urochordata (formerly Hemichordate) animals may resemble the ancestors of chordates. The vertebral column and skull are not formed in these animals.

In higher chordata animals like vertebrates animals central nervous system with brain & spinal cord, vertebral column and skull are formed along with other respective systems.

PHYLUM CHORDATE-CLASSIFICATION

Phylum Chordate is divided into three Sub-Phyla.

SUB-PHYLUM: 1. UROCHORDATA,

SUB-PHYLUM: 2. CEPHALOCHORDATA,

SUB-PHYLUM: 3. VERTEBRATA.

The first two sub phyla are called lower chordates or PROTOCHORDATES. They are usually called Acrania group. The vertebrata sub-phylum is called CRANIATA group.

SUB-PHYLUM-i: UROCHORDATA: The urochordate animals belonging to this sub-phylum re called sea-squirts. The adults are marine and mostly fixed. Free living forms are also present. The body is covered by test or tunicin. The notochord and nerve cord are found in the larval stage. in the adult form they are completely lost. Gill-slits may be permanent. They are’ bisexual forms. The larvae are free swimming.

Ex: Olkopleura, A.scidian, Herdmania, etc.

SUB-PHYLUM2 : CEPHALOCHORDATA: cephalochordata are marine organisms. Their body is fish-like with notochord and nerve cord persisting throughout life . They extend the entire length of the body. The eyes, and jaws are absent. The fundamental plan of the chordate body is seen in its most simple form in these animals. Gonads are paired in Amphioxus and unpaired in Asymmetron.

Ex: Amphioxus, Asymmetron.

SUB.PHYLUM 3 :- VERTEBRATA OR CRANIATA : vertebrata are well developed chordates. They show distinct head. Notochord is replaced by the vertebral column completely or partially. The nerve-tubes anterior and enlarged to form a brain. Cranium protects the brain. Visceral clefts called Gills perform respiration. They are not more than seven pairs. Heart is ventral Andes are present They are formed by several segments.

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 generaBranchiostoma and Asymmetron

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.

 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 iscommonly 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.

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.

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

1) Chilopoda :

1. Phylum Arthropoda includes this group Chilopoda.

2. Chilopoda includes centipedes.

3. These are dorso - ventrally flattened, distinctly segmented arthropods.

4. The body shows 2 divisions. 1. head and 2. long trunk.

5. The head bears a pair of long, many jointed, sensory antennae, a pair of   eyes, a pair of toothed mandibles and two pairs of maxillae.

6. The mouth lies on the ventral side near the anterior border of the head.

7. The trunk contains 15-173 segments in different species.

8. Each segment has a flat tergum on the dorsal side, sternum on the ventral side.

9. Each trunk segment, except the last two, bears on its lateral sides a pair of legs. They are modified into stout poison claws or maxillipedes. They are used for killing the prey.

10. The anus is situated at the extreme end of the trunk.

11. There is a single genital opening.

12. The genital opening opens in front of the anus on the ventral side of thepenultimate segment.

13. A pair of long Malphighian take up excretion.
14. Tracheae serve as respiratory organs.

15. Sexes are separate, there is a single gonad above the alimentary canal. Ex : Scolopendra.

2. Diplopoda :

1. Phylum Arthropoda includes this class.

2. The class diplopoda includes the millipedes,

3. These are long, cylindrical, segmented arthropods.

4. Body has 3 divisions, head, thorax and abdomen.

5. The head bears a pair of short, 7 jointed antennae, a pair of eyes, a pair of mandibles and a pair of maxillae.

6. The maxillae are fused with the first thoracic sternum to form a large labium like structure called the Ganthochilarium.

7. The mouth lies between the mandibles.

8. The thorax is short and consists of four single segments.

9. Each segment has an arched tergum and a small flat sternum.

10. The abdomen is long and comprises 20 to 103 segments.

11. Each segment bears two pairs of 7 jointed legs.

12. The third abdominal segment bears in the male a pair of short gonads or auxilliary compulatory organs.

13. There are two genital apertures that open out towards the anterior end on the third segment close to the second pair of legs (Progoneata).

14. A pair of long malphighian tubules take up excretion.

15. Tracheae will serve as respiratory organs.

16. Sexes are separate. Ex : Julus

 

also read ONYCHOPHORA –PERIPATUS-STRUCTURE-AFFINITIES

• A coral reef is a ridge in the sea with its upper surface nearer to the surface of water.
• This ridge is formed by calcium carbonate secreted coral organisms of cnidarians.
• These reefs are formed in warm, clear and shallow sea waters.
• Hence these reefs are confined to tropical regions only.
• These reef building organisms require 22 to 28°C and shallow depths up to 30 meters.
• There are 3 types of coral reefs.

1. Fringing reef  2. Barrier reef  3. Atolls reef

 

1. Fringing reef:

• It lies close to the shore.
• The reef may extend to a distance of 200metres from the shore into the sea. Its active growing region is called reef front.
• It is facing the sea.
• The narrow shallow water channel between shore and reef is called reef flat.
• This reef is largely composed of calcium carbonate, sand, dead, and living coral colonies and other organisms. 

  

 

2. Barrier reef:

• This type of coral reef is located some distance from the shore.
• The reef front is facing the sea.
• The reef and shore are separated by a deep stretch of water called lagoon. This lagoon is suitable for navigation.
• The best example for barrier reef is "Great barrier reef of north eastern Australia.
• It is 15 to 250 km away from shore. It extends up to 2000 km in length.

 

3. Atoll reef:

• It is also called coral island.
• It is a ring like or horse-shoe shaped reef that encircles a lagoon.
• The lagoon is very deep and is suitable for navigation.
• The ring like reef may be broken by a number of channels.
• This is called atoll.
• The biggest Atoll in the world is seen in the form of Suva diva of Maldives.

 

Theories explaining the formation of coral reefs:

Many theories were put forward to explain the formation of coral reefs.

 

1. Darwin - Dana subsidence theory:

1. Reefs start as fringing reefs on a shore slop.
2. This fringing reef by the subsidence of the reef flat into lagoon becomes barrier reef.
3. A fringing reef formed around an island becomes an atoll if the island is fully submerged.'
4. This theory is supported by Dana and Davis.

2. Submerged bank theory:

1. Because of the sedimentation, shallow slopes are formed, on which reefs are developed.
2. By the growth of the outer edges of the reef and dissolution of inner regions of the reefs barrier reefs and atolls are formed.

3. Daly Glacial-epoch theory:

1. In the last glacial period water was drawn from ocean to form ice caps .
2. Because of which the ocean level was lowered by 60 to 70 meters.
3. Rat slopes are formed, which are suitable for the development of reefs.
4. When there is a rise in temperature and with the rising of ocean level by melting of ice the reef had grown further.
5. Thus barrier reefs and Atolls are formed.
6. None of the theories gave satisfactory account of coral reef formation.

 

CORAL REEF IMAGES:
The following are the pictures of coral reefs.The first one is 1.The Great BarrierReef and the remaining are 2.Fringing Reef 3. Atolls Reef


 


  

CORAL REEFS AROUND THE WORLD

The following image shows coral reefs around the word in differentzoogeographical regions. In the below shown corals reef picture the red color dotted area indicates the coral reefs locations on the globe.

the previous topic is POLYMORPHISM IN COELENTERATE

 

Palaemon is a unisexual animal. Male and Female animals shows many morphological differences.

S.No.	Male	Female
1	The second pair of chelate legs are longer and stronger.	The second pair of chelate legs are smaller and thin,
2	The thoracic legs are closely arranged.	The thoracic legs are less closely arranged.
3	Second pair of pleopod has appendix masculina.	Absent.
4	The male genital openings are present on the arthroidal membrane of the last pair or walking legs.	The female genital openings are on the coxa of the third pair of walking legs.
5	The male prawn is bigger when compared with female prawn of the same age.	The female prawn is smaller than the male of same age.

Male reproductive system:

The male prawn has a well-developed male reproductive system, it has the following parts.

a) Testis: They are two in num­ber. They are long and narrow. They are present on the posterior half of the hepato-pancreas and beneath the pericardial sinus and heart. Anteriorly they extend up to the renal sac and posteriorly they run upto the first abdominal segment. In between the two testes a gap is present in the middle. Through this gap cardio-pyloric strand passes.

Each testis is composed of a many lobules and which are connected by connective tissue. The lobule is fined by germinal epithelium.

b) Vasa deferentia: From each testis a vas deferens arises, k is a long narrow duct. It coils near the hepato-pancreas. It runs vertically downwards.

c) Seminal vesicle: The lower part of the vas deferens is expanded. It is club shaped and muscular. It is called seminal vesicle. It stores the spermato-phores.

d) Male genital openings : The seminal vesicles open out through male genital pores. These openings are situated on the arthrodial membranes of the last pair of walking legs. Each opening is guarded by a small flap.

B. Female reproductive system:

It contains a pair of ovaries and a pair of oviducts.

a) Ovaries: These are present on the posterior half of the hepatopan-creas below the pericardial sinus and heart. They extend anteriorly up to the renal sac and posteriorly up to the first abdominal segment. In a mature female, the ovaries are sickle shaped and lie side by side, with a big gap between them in the middle. Cardiopybric strand passes through it. In the breeding season the ovaries enlarge and may extend into the first abdominal segment. Each ovary has a large number of ova surrounded by a membrane. The ova are in the different stages of development in the ovary.

b) Oviducts: These are slender and curved tubes. They have a wide proximal end. Each oviduct starts from the middle of the outer border of the ovary. It runs vertically downwards.

c) Female genital aperture: The oviducts open out through female genital opening. They are present on the coxa of the third pair of walking legs.

Copulation and fertilization: During copulation the male deposits the spermatophores near the female genital openings of the female. Near the female genital opening ova are present attached to the pleopods. Fertilization is external

Development: In Palaemon the devebpment is direct. The larval stage is absent. The eggs hatch and give small prawns which undergo many moultings before becoming adults.

Palaemon excretory system con­tains a pair of kidneys called the antennary or green glands and a median sac called renal sac.

I. Antennary glands: The antennary glands are present in the coxae of the antennae. Each gland is small, and white in color. Each gland shows three parts.

1. End - sac.

2. Labyrinth, and

3. Bladder.

1) End - sac :-It is small. It is the central part of the antennary gland. Its wall is folded inside. It has ephithelium and outer connective tissue. The cavity of the end-sac contains a large lacuna. It opens into the labyrinth by a pore.

2) Labyrinth : It is larger than the end sac. It is a mass of branched excretory tubules. They are held together by connective tissue. The labyrinth opens into bladder by several openings. The wall of each excretory tubule is formed by exeretory epithelium.

3) Bladder : Bladder is placed inner side of the end-sac. It opens into renal sac by the lateral duct. It gives excretory duct or ureter, from its inner side. It opens out by the excretory or renal apenure on the inner side pf the coxa of the respective antenna.

II. Renal Sac : It is a large median sac present beneath the dorsal shield. It covers the entire cardiac stomach. Anteriorly it is connected with the bladder of each antennary gland by a lateral duct.

Working : The green glands perform excretion and osmoregulation.

i) Excretion : The green glands are supplied with blood by the antennary arteries. By ultrafiltration water and dissolved substances pass into the end-sacs. The filtrate passes into the labyrinth. By selective reabsorption the useful products are returned to blood in labyrinth. The urine is collected and it flows into the bladders. The excretory fluid also comes here from the renal sac. The urine is passed out through the ureters and renal apertures.

ii) Osmoregulation : Water enters into the body regularly by osmosis. It is collected and sent out by green glands. Prawn passes out huge quantity of water with urine to maintain osmoregulation.

The nervous system of Palaemon resembles that of annelidas, it shows

1. Central nervous system.

2. Peripheral nervous system, and

3. Autonomic nervous system.

1. Central nervous system: It includes a pair of cerebral ganglia, a pair of circum esophageal connectives and a double ventral nerve cord.

i) Cerebral ganglia:

1. They lie in front of the junction of the oesophagus with the cardiac stomach. It is a white bilobed mass.
2. It is also called brain.

ii) Circum oesophageal connectives :

1. These are nerve cords which originate from the postero-lateral parts of the brain, run downwards round the oesophagus to unite with ventral nerve cord.
2. Each connective shows a small commissural ganglion which gives a mandibular nerve to mandible.

iii) Double Ventral nerve cord :

1. It lies in the mid-ventral line of the body, it bears seventeen pairs ganglia.
2. Tare anterior eleven pairs of ganglia belong to the cephalothorax and are fused to form a single and large mass called the Cephalo thoracic ganglionic mass.
3. The posterior six pairs of ganglia of the nerve cord belong to the abdomen and they remain separate.
4. The sixth and abdominal ganglion is larger than others.

1. ANTENNULARY NERVE

2. ANTENNARY NERVE

3. BRAIN

4.PERI-OESO PHAGEALL CONNECTIVES

5. SYMPATHETIC NERVOUS SYSTEM

6. VENTRAL THORACIC GANGLIONIC MASS

7. APERTURE FOR STERNAL ARTERY

8. OPTIC NERVE

9. TRANSVERSE CONNECTIVE

10. ABDOMINAL GANGLIA

2. Peripheral nervous system : It contains paired nerves that arise from the central nervous system to innervate body parts.

a) Nerves from brain: They are 5 pairs.

1. Optic,
2. Ophthalmic,
3. Antennulary,
4. Antennary and
5. Tegumental nerves.

1.The optic nerves extend outwards and forwards through the eye-stalks and innervate the eyes.

2.The opthalmic nerves arise close behind the optic nerves. They supply the oscular muscles in the eye stalks.

3. The Antennulary nerves arise from the brain antero-ventrally. Each nerve enters the antennule of its side. It also gives the statocystic nerve which goes to the statocyst.

4. The antennary nerves arise from .the ventral side of the brain. Each nerve enters the antenna of its side.

5. The tegumental nerves arise behind the antennary nerves. They innervate the labrum.

b) Nerves from Cephalothoradc ganglionic mass: Eleven pairs of nerves originate from the cephalo thoracic ganglionic mass.

1. A pair of Mandibular nerves to mandible.
2. A pair of Maxilhilary nerves to maxillulae.
3. A pair of Maxillary nerves to maxillae.
4. Three pairs of Maxillipede nerves to maxillipede.
5. Five pairs of walking leg nerves to walking legs.

c) Nerves from Abdominal ganglia:

1. Three pairs of nerves arise from each abdominal ganglion.
2. The fifth abdominal ganglion gives off only two pairs of nerves.
3. The sixth abdominal ganglion gives six pairs of nerves.

3. Autonomic nervous system:

1.  A slender nerve arises from the hind end of the brain and has two small visceral ganglia.
2. The anterior visceral ganglion is connected with the commissural ganglia of the circum-oesophageal connectives by short transverse connectives.
3. The posterior visceral ganglion gives two pairs of nerves to the muscles of the wall of the oesophagus and stomach.

SENSE ORGANS OF PALAEMON

Palaemon shows different sense organs. They are

1. STATOCYSTS,
2. TACTILE SETAE,
3. OLFACTORY SETAE,
4. COMPOUND EYES.

1) STATOCYSTS :

1. In palaemon a pair of statocysts can be seen.
2. The precoxa of each antennule will contain a statocyst. A statocyst is a round sac.
3. It is 1 to 1.5 mm wide. Its outer opening is covered by a small integument. Inside it an oval ring of setae, and heap of sand grains will be present.
4. A receptor setae has two parts 1 )a swollen base and 2) a slender shaft. The base is articulated with the wall of the sac by a membrane.
5. It is supplied with nerve from the statocystic nerve.
6. The shaft is bent and fine bristles are present above the bent part.

Working :

1. The statocysts are the organs which maintain equilibrium in the normal position. The sand grains lie on the floor of the statocyst.
2. The sand particleswork like statoliths.
3. When the position of the animal in water is changed the sand grains press against some of the receptor setae.
4. Nerve fibres are stimulated. It is conveyed to the brain. Position is corrected.

A) Statocyst in antennule B) Single receptor setae

C) Tactile setae D) Olfactory setae E) T.S. of Statocyst1) Precoxa
2) Statocyst
3) Antennary nerve
4) Sensory hair
5) Statoliths
6) Cuticle
7) Blade
8) Root

2) TACTILE SETAE:The tactile setae are seen on the antennae and on other parts of the body.

A tactile setae has two parts.

• Stout Proximal segment : Articulating with the integument by a flexible arthrodial membrane.
• Distal segment: It is touch sensitive. It is also called blade. It shows two rows of barbs.

3) OLFACTORY SETAE :

1. The olfactory setae, are on the middle small feeler of each antennule.
2. Each setae has a stalk and a blade.
3. Each setae gets a small nerve from antennary nerve.

4) EYES:

1. There are a pair of compound eyes.
2. They are stalked.
3. The eye stalks are in orbital notches near the base of the rostrum.
4. Each eye-stalk has two segments.

1. Proximal segment attached to the integument.

2. Distal segment with eye at its free end.

Structure : the compound eye consists of a large number of Ommatidia. They are radially arranged.

1) Dioptric part:

1. The eye is covered by a transparent comea.
2. The corneal lens is biconvex.
3. It is secreted by a pair of comeageal cells. Below these cells crystallirje cells are pressure.
4. The crystalline cone is secreted by four vitrellae cell.
5. All the above structures collectively form dioptric part of the ommatidium.

2) Retinal part:

1. Below the dioptric part a long, refractile rod, the rhabdome is present.
2. It is secreted and surrounded by seven long cells, the retinulae.
3. The end of each retinal cells is prolonged into a nerve fiber that joins the optic nerve.
4. The rhabdome and the retinulae from the retinal part of the ommatidium.
5. Each ommatidium is covered by pigmented cells which will separate it from the nearby ommatidium.

Function:

1. During bright light, ommatidia are completely covered by the pigment cells.
2. The ommatidia are separate from one another.
3. Thus many images are developed. Such-an image is called the oppositionor mosaic image and the eye is said to have a mosaic vision.
4. During dim light pigment cells separate apart and expose the ommatidia. With the result, the rays of light entering several adjacent corneae will overlap The image is formed by overlapping points of light.
5. Such an image is known superposition image.

Palaemon is a fresh water Prawn. It performs respiration by 3 organs.

1. Gills or Branchiae

2. Epipodites

3. Lining of branchiostegites

1. Gills or Branchiae :

1. There are eight pairs of gills.
2. They show semi-lunar shape.
3. They are arranged vertically one, behind the other in a row.
4. The third pair of gills are the smallest.
5. The gills are attached by their middle part to the body, which is called root.

According to their attachment three types of gills are seen in Palaemon.

i) Podo branches : The gills are attached to podomere.

Ex : The first pair of gills are attached to the coxae of the second maxiiii pedes.

ii) Arthro branches : These are attached to the arthrodial membrane which concects the appendage to the body.

Ex : The second and third pairs of gills are attached to the arthrodial membrane joining a limb with the body.

iii)Pleuro branch: The gills are attached to the lateral walls of the body.

Ex.- In Palaemon 5 pairs of gills are attached to the lateral walls of the thorax, one above each walking leg.

Structure of gill:

1. A gill consists of a narrow 'vertical axis' with two rows of thin "gill plates".
2.  The gill plates of each row are arranged like the leaves of a book.
3. Such gills are called phyDobranches.

1. Axis and Blood Supply:

1. The axis of the gill contains a central core of connective tissue.
2. It has three longitudinal blood channels. Two are lateral longitudinal channels.
3. The third one extends beneath are groove between the rows of gill plates and is called median longitudinal channel.
4. The lateral longitudinal channels are connected together at intervals by transverse channels.
5. The transverse channel present opposite to the root of the gill receives the afferent braftchial channel which brings deoxygenated blood from the body.
6. Each lateral channel gives off a slender marginal channel to each gill plate of its side.
7. It opens into the median longitudinal channel.
8. The median longitudinal channel gives of the efferent branchial channel that carries oxygenated blood to the pericardial sinus and the heart.

2. Gill Plate:

1. It is made by a single layer of cells.
2. A marginal blood channel is present in it.
3. The gill plate is covered by cuticle.

Working:

1. The balers [Scaphognathites] by their movement produce a constant current of water which passes through the gill chambers.
2. The current Of water enters the gill chambers from behind and passes upwards and leaves at the anterior end.
3. The lining of the gill plate has very thin cuticle which contains blood.
4. Therefore, the oxygen of the water diffuses into the blood and C0₂ from blood diffuses into the water.
5. The oxygenated blood returns to the heart is and supplied to the body parts.

2. Epipodites:

1. Three pairs of epipodites are present.
2. They are present in the anterior part of the gill chamber.
3. They are thin and leaf-like.
4. They are the outgrowths of integument of the coxae of the maxilhpedes.
5. The first pair of epipodites are larger and bi lobed.
6. The epipodites regarded as simple gills. They are supplied with blood.
7. They are in contact with water.
8. They take up respiratory function.

3. Lining of Branchlostegites :

1. The inner fining branchlostegites is thin. It contains blood lacunae.
2. It is in contact with fresh water.
3. Because of which lining of branchiostegite is respiratory in function.

PALAEMON-CIRCULATORY SYSTEM

In Palaemon the circulatory system includes blood, dorsal sinus, heart, blood vessels, blood sinuses, blood channels. The circulatory system is open type. The arteries open into blood sinuses in the body.

Blood:

1. It is colorless watery fluid.

2. It contains amoeboid cells.

3. The respiratory pigments is haemocyanin, hence the blood is bright blue in color when combined with oxygen. It is colorless when de-oxygenated.

4. Blood distributes digested food, oxygen to all body parts.

5. Blood has the capacity of clotting.

Dorsal sinus: The heart is dorsally placed in the posterior part of thorax. It is enclosed in a chamber called the dorsal sinus or pericardium. It is present above the hepato pancreas and the gonad. It shows a thin septum on the floor. The septum is attached to the dorsal body wall and to the thoracic wall.

Heart: It is a muscular organ. Its apex is directed anteriorly. Broad base is directed posteriorly.

Cardio-pyloric strand and two lateral strands will keep the heart in position inside the pericardium. On the wall of heart five pairs of valvular Ostia are present. Blood from dorsal sinus can enter into the heart through Ostia.

i) First pair of Ostia - Mid dorsal.

ii) Second pair of Ostia - Mid ventral.

iii) The third pair - Posterior.

iv) The fourth pair - Antero laterally,

v) The fifth pair-Postero laterally.

Blood vessels or arteries:

The heart sends blood to the body through the arteries. They are.

a) Median ophthalmic arteries: It arises from the apex of the heart. It runs forward mid-dorsally to head. It joins the two antennary arteries above the oesophagus in cephalic region.

b) Antennary arteries: A pair of antennary arteries arises from the apex of the heart on both sides of the median ophthalmic artery. Each artery runs forwards along the outer border of the mandibular muscle. Its branches are

1) Pericardial branch goes to pericardium.

2) Gastric branch goes to cardiac stomach in prawn digestive system.

3) A mandibular branch goes to mandibular muscle. Then it divides into a dorsal branch and a ventral branch. The ventral branch gives branches to antennule, the antenna and the renal organ. The dorsal branch gives branches to the eye and then divide. It unites with the same opposite branch and the median ophthalmic to form a circular loop like artery or circulus cephalicus. It gives a pair of rostral arteries to the-rostrum.

1) SUPRA INTESTINAL ARTERY

2) MEDIUM POSTERIOR ARTERY

3) OSTIUM

4) HEART

5) PERICARDIAL ARTERY

6) HEPATOPANCREATIC ARTERY

7)  12) ANTENNARY ARTERY

8) GASTRIC ARTERY

9), MANDIBULAR ARTERY

10) VENTRAL BRANCH

11) RENAL ARTERY

13) ANTENNULARY ARTERY

14) DORSAL BRANCH

15) OPTIC NERVE

16) & 18) ROSTRAL ARTERY

17) CIRCUKIR CEPHALICUS »

19) STERNAL ARTERY

20) VENTRAL THORACIC GANGLIONIC MASS.

21) VENTRAL ABDOMINAL ARTERY

22) VENTRAL THORACIC ARTERY.

Hepatic arteries : They arise' from heart ventro-laterally one on each below the antennary artery. They go to the hepatopancreas.

Median posterior artery : A short stout artery arises from the postero -ventral surface of the heart. It bifurcates into a supra-intestinal artery and a sternal artery. The supra intestinal is also called dorsal abdominal artery which supplies blood to the midgut and the dorsal abdominal muscles. The sternal artery runs downwards through the hepatopancreas and then pierces the ventral thoracic ganglionic mass and go to the ventral side. It divides into ventral thoracic branch and ventral abdominal branch. The ventral thoracic branch supplies blood to the sternal region, the first three pairs of walking legs, the maxillae and the maxillulae. The ventral abdominal branch runs posteriorly up to the anus and supplies blood to ventral abdomen, the last two pairs of legs, the hindgut etc.

Blood sinuses : True capillaries and veins are absent. In Palaemon arterial branches end in blood sinuses of the haemocoel. All the sinuses of the body open into a pair of ventral sinuses. It is present below the hepatopancreas.

Blood channels: These have no proper walls. Six afferent bronchial channels carry blood from ventral sinus to the gills on both the sides. Oxygenated blood from the gills of both sides is brought to the pericardium by six afferent bronchial channels.

Circulation of blood: The heart because of contractions forces the blood through the arteries to all parts of the body. Blood from all sinuses will return to two ventral sinuses. From there the blood is distributed to the gills by afferent bronchial channels and then it reaches the pericardial sinus through efferent bronchial channels and reach heart.

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.

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.

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.

Urochordata General Characters

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. AllUrochordates 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.

Classification of Urochordata
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: 1. 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. ‘

Affinities of Urochordates .

The typical tunicate looks like a non-chordate animal. If the life history of such animal is studied, the larval form reveals the chordate chara’4ters 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 ofthe 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. But the

Urochordates differ with other Chordates because of the following peculiar characters.

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.

External characters of Herdmania

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 Ls 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.

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. Mesènchyme: 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.

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.

NERVOUS SYSTEM OF HERDMANIA:

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.

Excretory Organ Of Herdmania:

A neural gland is present above the brain in herdmaina. it Is brown in colour. It is present In the mantle. 1t is 4mm in diameter. It show branching tibuIes. At one end it leads into a short duct which opens by ciliated funnel above the dorsal tubercle. In the blood nephrocytes cell are present. They coiled waste matter. They come to the neural gland from there, they are sent out The neural gland secretes a hormone. It help in metamorphosis. Neural gland is considered homologous to pituitary glands of vertebrates.

Herdmania- Spicules in the test:

In the test of Herdmania two types of calcareous spicules are Present. They are:

1) Microscieres : These are 40 to 80 microns in length. They are minute.

2) Megascleres : These are long spicules. They show different shapes. They are 1.5 to 3.5 mm in length. They are two types.

a) Spindle shaped Megascleres. They are 1.5 to 2.5 mm in length.

b) Pipette shaped Megascleres. They are 3.5 mm. in length.

The rnicroscleres are present only in the test. But the megascleres occur in the test, body wall, and viscera.

Herdmania-Spicules Function:

1) They give support to the test

2) They protect the animal from predators.

3) They fix mantle with test.

Retrogressive Metmorphosis in herdmania

During metamorphosis the larva will loose all the chordate characters and attains an invertebrate like form. This type of metamorphosis, where highly advanced larval form ends in a lowly organised adult is called retrogressive metamorphosis.

Fixation of the larva: The larva swims for some time without feeding. It is fixed to a sub- stratum with the help of the adhesive papillae. It stands erect with the tail upwards. Then it undergoes retrogressive metamorphosis.

Changes during metamorphosis:

1.Notochord, nerve cord muscles and tail will be reduced. All the above structures will help the larva to swim freely in the water. But they are not useful to the sedentary adult

2.. The alimentary canal becomes complicated. The pharynx en larges ln size. The number of gill slits will increase by divisions. The stomach and intestine will grow.

3. The nervous system is reduced and the anterior part of nervous system is developed into a small neuralganglion attached to it neural gland is present. ;:

4. The atrial cavity enlarges into a sac like structure.

5. The eyespot and statocyst will completely disappear.

6. Gonads develop from mesencyme.

When these changes are taking place, the region between the adhesive papillae and mouth grows very rapidly. At the same time the growth of the dorsal region is stopped. Because of this, the body rotates through 1800 angle and mouth is brought to the top

ORGANISATION OF HERDMANIA TADPOLE LARVA:

Herdmania is a hermaphrodite animal. The fertilised eggs undergo holoblastic unequal clevage and it develops into blastula. it shows upper micromeres and lower macromeres. By invagination of the macromeres gastrulation takes place and gastrula is formed. This gastrula develops into a tailed larva called Ascidian Tadpole larva. (Herdmanis life history, is not clearly known. Clavilina’s life history is known. It is followed here.

The larva is 3 mm in length. It has short oval body and a long tail.

This larva shows all the chordate features.

1) The body is covered by thin test.

2) The tail is long and shows a tail fin or caudal fin.

3) The tail is supported by notochord. Hence it comes under urochordata.

4) On the dorsal side above the notochord hollow nerve cord is present. This nerve cord is enlarged at the anterior end as a cerebral vesicle. In the cerebral vesicle pigmented eye spot is present. Statocyst is also present. They work as sense organs.

5) On either side of the notochord in the tail region muscles are Present which are helpful in the locomotion.

6) On the trunk region digestive system is present. It shows large pharynx with few gills slits. They open into atrium. On the mid ventral floor of the pharynx an endostyle is present.

7) Atrium opens out through atriopore.

8) Below the pharynx on the ventral side a muscular heart is present.

9) On the anterior end of the trunk three adhesive papillae are present These are very much useful to attach the larva to the substratum

This Herdmania tadpole larva shows all chordate characters. This larvae ‘undergoes retrogressive’ metamorphosis and develops into adult Herdmania.

Gonads of Herdmanla :

Ans: Herdmania is a bisexual animal. It is a protogynous animal. Ovary matures’ first. Hence cross fertilization takes place.

Herdmania contains a pair of gonads. The left gonad lies in the intestinal loop above the heart. Each gonad shows 10 to 25 lobes arranged in two rows. The median lobe is single and large. Other lobes are oval in shape.

Each lobe shows outer large testicular part and inner small ovarian part. The testicular part is brick red in colour and produces sperms. The ovarian part is pink in colour and produce ova. From each testicular part sperm ductule will arise. They open into spermduct. From each ovarian part ovarian ductule will come. They open into the oviduct. Each gonad has an oviduct and spermduct. They run parallelly and open separately into cloaca behind anus.

CYCLOSTOMES-GENERAL CHARACTERS

Cyclostomes’ are jawless vertebrates, They are primitive vertebrates. The cyclostomes are marine or fresh-water vertebrates.

It includes the lampreys and hag fishes.

1) The body is long, eel like. It shows trunk and compressed tail.

2) Paired fins are absent. Median fin is supported by cartilaginous fin-rays.

3) The skin Is soft and smooth. It is slimy. It as scale less.

4) Z shaped myomeres are present in the trunk and tail Protractor and retractor muscles move the tongue.

5) In this group a true coelome is seen.

6) These vertebrates will not have jaws, hence called Agnatha.

7) The mouth is circular. It works like a sucker. It is surrounded by tentacles.

8) Tongue bears teeth.

9) Stomach is absent and oesophagus leads into the intestine.

10) Endoskeleton is present.

11) Skull is simple and primitive.

12) Notochord presists throughout life.

13) Vertebrae are represented by neural arches, around the notochord.

14) Five to sixteen pairs of gills are present in sac like pouches

15) The heart is two chambered. Sinus venousus is present, but conus arterlcsus is absent.

16) Blood contains leucocytes and Irregular nucleated erythrocytes..

17) Brain is seen.

18) Ten pairs or less number of cranial nerves are present.

19) Nasal sac is single and median.

20) Lateral line sense organ is present.

21) Excretory system includes a pair of mesonephric kidneys.

22) Sexes are separate.

23) Gonad is single and without a gonoduct.

24) Development may be direct or with a long larval stage.

CLASSIFICATION OF CYCLOSTOMATA

The class Cyclostomata is divided into two orders:

 

1) PETROMYZONIA AND 2) MYXINOIDEA

.

Order 1. Pefromyzonia

1)Lampreys are included In this order.

2) The buccal funnel is suctorial and shows horny teeth.

3) The mouth is present in the buccal funnel.

4) The nasal sac is dorsal. It has no connection with the pharynx.

5) Eyes are functional.

6) Seven pairs of gill slits are present.

7) A well developed dorsal fin is present.

8) Branchial basket is complete.

9) Brain is well developed.

10) Pineal eye is well developed.

11) Ear has two semicircular canals.

Ex: 1. Petromyzon (Sea-lamprey) ) Lampetra fluviatilis.

Order 2: Myxinoidea

1) It includes the hag-fishes or slime eels.

2) Buccal funnel is absent.

3) The nasal sac open into pharynx, through a canal.

4) Eyes are vestigial organs.

5) Dorsal fin is absent or very small.

6) Branchial basket is poorly developed.

7) Brain is primitive.

8) Pineal eye is reduced.

9) Ear has only one semicircular canal.

10) The hag-fishes are all marine.

Ex: 1. Myxine glutinosa Hag-fish or slime eel). 2. Eptatretus (Bdellostoma)

BRANCHIAL BASKET

In Petromyzon the visceral skeleton contains branchial basket. It liesexternal to the gill pouches. It shows 9 irregular vertical bars and 4 longitudinal bars on either side. The first vertical bar lies just behind the styloid cartilage. The second is in front of the first gill slit. The remaining seven vertical bars are behind the 7 gill slits. The last vertical bar Is joined to a cup like pericardial cartilage. It supports the posterior and lateral walls of the pericardium. The longitu1nal bars are called subchordal, epitrematic, hypotrematic and median ventral. The subchordal bar runs along the notochord. Ft is joined to the cranium In front. The epitrematic and hypotrematic bars lie above and below the gill slits. The median ventral bar is placed in the midventral line and partly fused with the corresponding bone of the other side.

 

Myxine (Hagfish)		Petromyzon (Lamprey)
I Hag fishes are scavengers. They penetrate into the dead fishes bodies and eat the tissues.		1.They are semi parasitic. But the Larvae(Ammocoetes) leads an independent life.
2. The body is. slender & eel-like		2. The body is cylindrical & stout.
‘3 They are exclusively marine habitats.		3. These live in both in sea water and fresh water. They migrate into the rivers for spawning.
4. the caudal fin is small.		4. The caudal fin is well developed.
5. The dorsal fin is not fully developed.		5. The dorsal fin is divided into two portions by a notch.
6. Mouth is terminal in position. It is surrounded by soft lips. Buccal funnel is absent.		6. Mouth is sub-terminal in position. Buccal funnel is present.
7. Single nostril is present very near to the mouth.		7. Single nostril Is on the dorsal side of head region.
8. Skull has no roof.		8. Skull roc is incomplete.
9. Small teeth are present on the tongue in two rows.		9. Horny teeth are present in the buccal funnel and on the tongue.
10. Salivary glands are present.		10. Paired salivary glands are present
11. The gill pouch number varies from six to fifteen pairs.		11. There are seven pairs gill- pouches present. All these open Into the respiratory tube.
12.Each aortic ‘arch supplies the hemibranch of a single gill pouch.		12. Each aortic arch divides and the branches enter the posterior hemibranch of the nearby gill pouch.
13. Brain is degenerated.		13. Brain is well developed.
14. Only thee pairs of cranial nerves are present.		14. Ten pairs of cranial nerves are present
15. Non functional paired eyes are burned in the skin.		15. Paired functional eyes are present.
‘16. Penial eye is absent.		16. Penial eye is present.
17. In the adult pronephros is retamed. But the functional kidney Is of mesonephric type and simple.		17. The functional adult kidney is mesonephric type.
18. UrinogenitaL sinus is not seen:		18. Urinogenital sinus is formed.
19. Sexes are united - Hermaphrodite. The anterior part of the gonad acts as ovary & the posterior part act Like Testis.
20. The eggs are large and covered by cylindrical horny shell,		20. The eggs are small and are not covered by horny shell.
21. The development is direct		21 Development is indirect. Arnmocoetes larva occurs in the life-history.

AMMOCOETES LARVAE-MATAMORPHOSIS

In the life history of Petromyzon a larval form is seen. It is called Ammocoetes larva.

1) Ammocoetes larva is transparent and 1 cm., in length.

2) Ammocoetes larva lives in U shaped burrows of mud.

3) Ammocoetes larva comes out of the burrow at night only to change its feeding ground, then it buries again.

4) Ammocoetes larva has an eel like body and it differs from the adult In several characters.

5) It has no buccal funnel.

6) Around the mouth a semi circular oral hood is present

7) Below the mouth short transverse lower lip is present and it has no teeth.

8) Paired eyes are covered by thick skin.

9) Behind the buccal tentacles, a velum with a pair of cup-shaped muscular flaps are seen.

10) Surrounding the mouth buccal tentacles are present.

11) The pharynx is associated with seven pairs of gill pouches.

12) The pharynx is continued into the oesophagus posterioily.

13) The endostyle is present on the ventral side of the pharynx.

14) The mucus secreted by the endostyle passes into the peripharyngeal grooves of the pharynx.

15) Food particles like unicellular algage, bacteria which enter with the incurrent of water and entangled with the mucous. Thus the food associated with mucous carried into the oesophagus from the pharynx. Because of muscular movement of velum and pharynx, the ammocoetes larvae feeds.

 

METAMORPHOSIS OF AMMOCOETES LARVA

1) After a long period of larval life, the Ammocoetes larva metamorphoses into the adult.’

2) The endostyle modifies into the thyroid gland.

3) The oral hood changes into the buccal funnel with horny teeth,tongue and a round mouth.

4) Paired eyes are developed completely.

5) The velum degenerates by leaving a rudiment.

6) The continuous dorsal fin breaks into dorsal and a single caudal fin.

7) The young lamprey migrates to the open sea from rives and becomes carnivorous adult animals.

The ammocoetes larva exhibits striking similarities with Amphioxus. it is a very primitive and generalised vertebrate. It may be regarded as the connection link between Amphioxus and cyclostomes