CHORDATES- ORIGIN-CHARACTERISTICS-CLASSIFICATION

 

CHORDATE DEFINITION

All animals having notochord are referred to as chordates. The phylum chordate-definitionchordate was established by Balfour in 1880. This phylum includes animals such as ascidians, lancelets, fishes, amphibians, reptiles, birds and mammals including man. Most of the chordates are free living and none is strictly parasitic. There are some 65,000 known living chordates besides the fossil remains of many extinct forms. However these animals exhibit some common characters which show their common ancestry.

ORIGIN OF CHORDATES AND EVOLUTION

The chordates were derived from a common ancestral stock. The ancestral chordates have appeared in the early part of Palaeozoic era. There are different opinions about the origin of chordates.

Echinoderm theory:

Garstang was of the opinion that chordates were evolved from the auricularia larva of Echinodermata.
The chordates are closely related to the echinoderms and hemichordates. It is evident by the following resemblances.
origin-of-chordates
1. The phosphogens used in muscle contraction are similar in hemichordates and echinoderms. The phosphogen is in the form of creatine in all vertebrates and arginine in invertebrates. In hemichordates and echinoderms both creatine and arginine are present.
2.The tornaria larva of hemichordates resembles the larval forms of echinoderms. It is believed that the ancestral chordates might have evolved from the larvae of echinoderms by paedogenesis
Chordata is the highly evolved phylum of the animal kingdom. The largest vertebrate animal is blue whale, Balaenoptera musculus. It grows upto 35 meters length and weighs about 120 tons.
The whale shark, Rhincodon typhus is the second largest vertebrate. It grows to 20 meters length.
 

CHARACTERISTICS OF CHORDATES

NOTOCHORD
1. It is a stiff elastic mid-dorsal longitudinal rod lying between the nerve cord and alimentary canal. So, it is called Chordadorsalis.
2. It is a skeletal or supporting structure present in the embryonic stages of all chordates. It is made of vacuolated cells and enclosed by two connective tissue sheaths.
3. It persists throughout life in Amphioxus, and vertebrates like cyclostomes.
4.Notochord serves as an organ of internal support and also provides surface for the attachment of muscles.
5.In vertebrates, the notochord is replaced by vertebral column. Hence all vertebrates are chordates but all chordates are not vertebrates.
6.The notochord is chorda mesodermal in origin. In mammals, notochord is in the form of vestigial swellings in the vertebral column. They are called Nuclei pulposi.
2. DORSAL TUBULAR NERVE CORD
The nerve cord in chordates is a single hollow tubular structure, situated dorsal to both the alimentary canal and notochord. During the development, it is formed from the mid dorsal region of the early embryo from ectodermal cells i.e., It is derived from embryonic neurectoderm. It persists through out life in most chordates, but in higher chordates, the nerve cord is differentiated into an anterior brain and a posterior spinal cord. The nerve cord gets degenerated during metamorphosis in ascidians.
3. GILL SLITS OR PHARYNGEAL SLITS
Gill clefts are paired openings leading from the pharynx to the exterior. Out pushing’s of the endoderm lining of pharynx meet and fuse with ectodermal in pushing’s from the exterior, the intervening walls being broken down, thus gill-clefts are formed. Such gill-clefts appear during the development of every chordate, but in many aquatic forms they are lined with vascular lamellae which form gills for respiration. In terrestrial chordates which never breath by gills, traces of gill-clefts are present during early development but most of them disappear before adult life.
Gill- clefts are also called gill – slits or pharyngeal – clefts. Such gill – clefts which do not bear gills are usually called visceral – clefts. In lower chordates the visceral – clefts often form some endocrine glands.
Even though the three primary characters of chordates are well developed, the dorsal tubular nerve cord is reduced in some and in urochordata the central nervous system degenerated in the adult. However the three primary characters are possessed only by chordates.
Other Chordate Characters:
Higher chordates also possess the following characters:
1.Bilateral symmetry
Various organs of the body are symmetrically arranged on right and left sides of the median longitudinal axis. In a bilaterally symmetrical animal, the body can be divided into two equal halves only in one plane (sagittal) which may be the mirror images of each other.
2. Triploblastic condition
Various organs in the chordate body are derived from three germ layers, namely ectoderm, endoderm and mesoderm. Thus, all chordates are triploblastic animals. In chordates, the mesoderm is formed as out pushings of archenteron, But in higher invertebrates the mesoderm arises as solid out growths from cells which lie at the juncture of ectoderm and endoderm.
3. Coelom
A true coelom or body cavity which develops from mesoderm is present in all chordates. But it is called enterocoelus coelom in chordates, as it develops from archenteron. The coelom is externally lined by parietal or somatic layer and internally by visceral or splanchnic layer. In higher chordates like mammals, the coelom is divided into anterior thoracic and posterior abdominal cavities by a diaphragm.
4. Metamerism (Segmentation) The linear repetition of the body organs is called metamerism. Among the chordates the metamerism is visible only internally. The body musculature, the nervous system, the circulatory system, the excretory system, etc., are atleast, at the time of origin thoroughly segmented.The segmentation in chordates is referred to as heteronomous segmentation, as the segmentation is not uniform.
Metamerism is clearly seen in the embryo, but it becomes obscured by condensation in the adult in some organs such as the kidneys and limbs and it is completely lost in the head.
5. Cephalization
All the higher chordates possess well developed head having complex brain and specialised sense organs. This trend towards the prominence of head is called cephalization.
In the chordate series there is a steady increase in size and specialization of the head. Thus there is greater prominance and domination of the head over the rest of the body.
6. Circulatory system As the blood flows through a system of tubes, the blood vascular system is of closed type. In dorsal vessel the blood flows from anterior to the posterior end and in the ventral vessel from posterior to anterior.
7. Ventral heart
The heart is a muscular and contractile organ situated towards ventral side inbetween the lungs below the oesophagus. The heart is surrounded by double walled pericardium. It is 2 to 4 chambered in various groups of vertebrates.
8. Hepatic portal system
Hepatic portal system is present in all the chordates. Blood that is collected from various parts of alimentary canal is not carried directly to heart but to liver. The blood then goes to heart. Thus the hepatic portal vein not only begins but also ends in capillaries or it acts both as afferent and efferent vessel.
9. Red Blood Corpuscles
In higher chordates, the respiratory pigment haemoglobin is always
found in the specialised cells called red blood corpuscles or
erythrocytes.
10. Paired appendages: Higher chordates posses two paired appendages in the form of fins or limbs.
11. Post tail :It is a posterior prolongation of the body. It is without coelom and viscera. But it has extensions of muscles, nerve cord, notochord.
CHORDATA PHYLUM-CLASSIFICATION
According to Balfour, Phylum Chordate is divided into three subphyla. They are:
1. Urochordata 2. Cephalochordata 3. Vertebrata or Craniata
Of these three subphyla, the first two and also Hemichordata are generally referred to as protochordates. All protochordates are characterised by the absence of cranium, so they are also called acraniates. The protochordates stand between invertebrates and vertebrates.
chordates-classification
Bateson included Hemichordata in this phylum. Now a days, the group Hemichordata is treated as a separate minor phylum.
Subphylum 1: Urochordata (Tunicata)
(Gr. uro = tail, chorda = notochord)
The urochordates are generally referred to as ascidians or sea squirts.
Exclusively marine. Some are sedentary and some are pelagic forms i.e., floats on water.
These are degenerated chordates. Their body is enveloped by a tunic or test made up of tunicin (cellulose like material). So they are called tunicates.
4. In the larval form, notochord is present only in the tail region, hence the name urochordata. It is absent in the adult stages. But in Larvacea, the notochord is present in adults also.
5.Dorsal tubular nerve cord is found only in the larval stage, but degenerates in the form of small ganglia in adults.
6. Numerous gill slits are present.
7. Heart is ventral, simple and tubular.
8. Coelom is completely absent.
9. Sexes are united i.e., hermaphrodite.
10. The larval form is called Tadpole and it exhibits retrogressive metamorphosis. The transformation of well-organized larva with all the chordate characters into a degenerated and less developed adult is called, retrogressive metamorphosis.
Urochordata has been divided into three classes.
1.Larvacea 2.Acidiacea 3.Thaliacea.
Larvacea:
1. Free swimming pelagic forms.
2. Neotenic forms which retain the larval form throughout adult life.
3. Test is temporary.
4. Posterior part of the body takes the form of a large locomotory appendage, the tail.
5. Single pair of gill-slits is present.
6. Anus opens ventrally on the surface of the body.
7. Sexes are united, i.e., hermaphrodite.
8. No metamorphosis.
Ex: Appedicularia, Fritallaria, Oikopleura.
Ascidiacea:
1. Fixed or free – swimming marine forms.
2. Simple or compound, solitary or colonial.
3. Adults are never provided with a locomotory appendage or tail and have no trace of notochord.
4. Test is permanent and well developed.
5. Branchial sac is large and well developed with its walls perforated by numerous gill-slits.
6. Reproduction both asexual and sexual.
Ex: Herdmania, Ciona, Ascidia, Botryllus.
Thaliacea:
1. Free swimming pelagic forms.
2. Solitary and colonial.
3. Test is permanent and transparent, may be slightly or well developed.
4. Musculature of body wall is in the form of circular bands.
5. Branchial sac has either two large or many small gill – slits.
6. Tail and notochord absent in adults.
7. Life history exhibits an alternation of generation. Ex: Pyrosoma, Salpa, Doliolum.
Subphylum 2: Cephalochordata
(Gr. cephalo = head, chorda = notochord)
1. Cephalochordates are commonly called lancelets.
2. These are small fish-like transparent, marine animals.
3. Pharynx is large with numerous gill-clefts opening not to the exterior but into ectoderm lined atrium.
4. Nerve cord is tubular and dorsal. No brain.
5. Segmentation and coelom are well developed.
6. Notochord is extended from the posterior end tothe anterior end (the tip of the snout) and hence the name cephalochordata.
7. Nephridia are the excretory organs.
8. Myotomes, nephridia, gonads and gill slits are metamerically arranged.
9. The pharynx has an endostyle and ciliated tracts which are concerned with ciliary feeding.
10. Cephalochordata are closely related to urochordata, both have been derived from a common ancestral stock.
Eg: Amphioxus, Asymmetron.
Sub-phylum 3: Vertebrata or Craniate
1. These are highly evolved, complex chordates with distinct head.
2. Notochord is partially or totally replaced by a vertebral column. Such animals are called vertebrates.
3.The anterior part of the nerve cord is enlarged to form the brain.
4. A cranium or brain box encloses and protects the brain, hence called craniata.
5. Visceral clefts are not more than 7 pairs except in some cyclostomes. Gill pouches do not open into an atrium.
6. Two pairs of appendages which help in locomotion are present in the form of fins or limbs.
7. Endoskeleton is cartilaginous or bony or both, derived from mesoderm. Exoskeleton is in the form of scales, dermal plates, feathers, hairs, hooves, horns, nails and claws.
8. Lungs are the respiratory organs except in fishes and some amphibians.
9. Respiratory pigment is haemoglobin which is found in erythrocytes.
10. The heart is ventral in position and is 2 chambered in fishes,’3 chambered in amphibians and 4 chambered in birds and mammals. In reptiles, incompletely divided 4 chambered heart is present.
11. Hepatic portal system is present in all vertebrates. Renal portal system is present in all vertebrates except mammals.
12. Liver and pancreas are found in association with digestive system.
13.Endocrine glands which pour their secretions into blood are present in all vertebrates.
14. Kidneys are excretory organs.
15. Post anal tail is present in vertebrates. It is vestigial in man and apes.
16. The brain is complex and there are at least ten pairs of cranial nerves.
17.Paired eyes and auditary organs are present.
18. Single pair of gonads is present.
19. There is no larval form. So the development is direct. Sub-phylum Vertebrata is divided into 2 superclasses. 1. Agnatha and 2. Gnathostomata
Super class 1. Agnatha (Monorhina)
(Gr. A = not, gnathos = jaws)
1. This super class is characterized by the earliest vertebrates without jaws and paired appendages.
2. A median single nostril is present.
3. Persistant notchord with a fibrous neural tube.
4. Presence of large number gill slits, from 7-14 pairs.
5. Absence of conus in the heart.
Superclass Agnatha is divided into
2 classes.1.Ostracodermi and 2.Cyclostomata
.
Class 1. Ostracodermi
(Gr. ostrakon = shell, derma = skin)
1. These are the most primitive, oldest and extinct jawless vertebrates (fossils) .
2. Fish like bodies with heavy head amour.
3. They had heavy bony dermal plates in the skin.
4. A slit like mouth at the extreme end of head. It was used for scooping decaying matter from the floor of the lake.
5. Median penial eye was also present.
Ex: Cephlaspis, Hemicyclospis, Clearas pis.
Class 2: Cyclostomata or Agnathostomata
(Gr. cyclo = round, stoma = mouth)
1. These are living fish-like jawless vertebrates.
2. Paired fins and scales are absent.
3. Endoskeleton is cartilaginous.
4. Heart is 2 chambered and aortic arches are many.
5. Mouth is round and suctorial.
6. Notochord is persistant throughout life.
7. They lead ecto parasitic life on fishes.
8. 6-14 pairs of gill slits are present.
9. Single gonad without duct.
10. Development may be direct or indirect. The larval form in lampreys is called Ammocoetus which resembles Amphioxus.
Eg: Petromyzon (Lamprey), Myxine (Hag fish), Bdellostoma.
Super class 2 : Gnathostomata (Amphirhina)
(Gr. gnathos = jaws, stoma = mouth)
1. These are the advanced vertebrates with a pair of true jaws.
2. Paired appendages and paired nostrils are present.
3. The endoskeleton is well developed.
This superclass is divided into 7 classes. They are
1. Placodermi, 2. Chondrichthyes, 3. Osteichthyes,
4. Amphibia, 5. Reptilia, 6. Aves,and
7. Mammalia.
The animals of the classes Placodermi, Chondrichthyes and Osteichthyes are collectively called Pisces. The animals of the remaining four classes are known as tetrapods because they have 2 pairs of pentadactyle limbs.
In fishes and amphibians, the extra embryonic or foetal membranes are not formed during development, hence they are called Anamniota.
In the development of reptiles, birds and mammals, an extraembryonic membrane viz. amnion is formed hence they are called Amniota.
Pisces, amphibians and reptiles are cold blooded or poikilothermic or ectothermal vertebrates i.e., they cannot maintain constant body temperature. Aves and mammals are warm blooded or homeothermic or endothermalvertebrates as they maintain constant body temperature due to thermoregulatory mechanism. Pisces and amphibians are included in the group Ichthyopsida,where as reptiles and aves are grouped into Sauropsida.


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