Here is a compilation of essays on ‘Chordata’ for class 11 and 12. Find paragraphs, long and short essays on ‘Chordata’ especially written for school and college students.
Essay on Chordata
Essay Contents:
- Essay on the Introduction to Chordata
- Essay on the Origin and Ancestry of Chordata
- Essay on the General Characters of Chordata
- Essay on the Fundamental Chordate Animals
- Essay on the Characters Common to Chordates and Higher Non Chordates
- Essay on the Subdivisions of Phylum Chordata
Essay # 1. Introduction to Chordata:
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The name of phylum Chordata is derived from two Greek words, the chorde (= a string or cord) and ata (= bearing). The common characteristic feature of phylum Chordata is the presence of a stiff, supporting rod-like structure along the back, the notochord (Gr., noton = back; L. chorda = cord) which is found in all the members of the phylum at some stage of their lives. Thus, chordates are those animals which have a cord, i.e., notochord.
The animals belonging to all other phyla of the Animal Kingdom are often called “the nonchordates” or “the invertebrates” since they do not have notochord or backbone in their body structure. The phylum Chordata was created by Balfour in 1880. The phylum Chordata includes primarily the Protochordata (Acraniata) and Vertebrata (Craniata).
Chordate Animals:
Phylum Chordata is the largest of the deuterostome phyla. It is the highest and most important phylum comprising a vast variety of living and extinct animals. Most of the living chordate animals are well-known familiar vertebrate animals such as the fishes, amphibians, reptiles, birds and mammals. The less well-known animals include a number of marine forms such as the tunicates and lencelets.
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Diversity of Chordates:
The chordates exhibit great diversity of form (anatomy), physiology and habit. The phylum Chordata includes a vast group of diverse animals ranging from Ascidian to Man.
Number (Numerical Strength):
The number of chordate species is not large. About 49,000 species are on record which are only half of the living species of molluscs, and less than one- tenth those of arthropods. The two subphyla Urochordata and Cephalochordata include nearly 2,500 species. The subphylum Vertebrata includes 46,500 species. Among the vertebrates, fishes are the most numerous with approximately 25,000 species. The approximate number of species of amphibians is about 2,500, reptiles 6,000, birds 9,000 and mammals 4,500.
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Size:
The urochordates and cephalochordates are mostly of small size. The vertebrates include animals of medium to large size, notably some huge sharks and whales. The gigantic blue whale (Balaenoptera musculus), which is about 35 metres in length and 120 tons in weight, is the biggest known animal. The whale shark (Rhineodon typus) attains a length of 15 metres and is the second biggest vertebrate after whales. The smallest fish is a Philippine goby (Pandaka) measuring only 10 mm long.
Ecology:
The chordates are not only the largest animals existing today, but ecologically they are among the most successful in the Animal Kingdom. They occupy various kinds of habitats and have adapted themselves to various modes of living than any other group, including the arthropods. Chordates are found in the sea, in freshwater, in the air and all parts of land from the equator to the poles. Birds and mammals are found in cold climates because they have a constant body temperature, which other animals do not have.
All lower chordates (protochordates) are marine, fishes are aquatic (freshwater and marine both) and higher chordates are predominantly terrestrial. Amphibians are not found in salt water. Birds do not live permanently in water, while some amphibians, reptiles and mammals permanently occupy the water. Most tunicates are sessile, all chordates are free-living and none is strictly parasitic.
Essay # 2. Origin and Ancestry of Chordata:
The chordates include organisms having a notochord, a dorsal tubular (hollow) nerve cord, pharyngeal gill-slits and a few features such as axiate organisation, bilateral symmetry, triploblastic condition, coelom, metamerism (segmentation), organ-systems, etc., that are common with the nonchordates. The question of the origin of chordates still remains unanswered and considerable controversy exists on this issue. Scientists have not succeeded in determining which lower forms have given rise to chordates.
Their early ancestors most likely were soft-bodied and left no definite fossil remains. The geological records established beyond doubt that the chordates originated prior to Cambrian period as the oldest fossils of known vertebrates have been discovered in Cambrian strata.
This fact has been emphasised by many workers on this line. Most scientists consider that the chordates have originated from invertebrates. Several theories have been proposed to explain the origin of chordates from nonchordate groups, but these theories have serious drawbacks and far from satisfactory.
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One theory advocates the origin of chordates from Echinodermata as such. Echinoderms are radially symmetrical animals and show no typical chordate characters, yet they seem to be most closely related to chordates. The auricularia larva of echinoderms is a free-swimming bilaterally symmetrical larva, and it very closely resembles the tornaria larva of hemichordates. But in recent years the ancestry of chordates from echinoderms source is not accepted.
Most zoologists, such as Romer, Berril and Barrington, etc., now favour the deuterostome line of chordate evolution, according to which the phyla Echinodermata, Hemichordata and Chordata show common ancestry on embryological and biochemical evidences. The protochordates provide the connecting link between early chordate ancestors and vertebrates. The differentiation probably occurred much earlier than Cambrian period.
The earliest traces of vertebrates have been found in the rocks of the late Cambrian period and Ordovician period. A number of fishes followed in Silurian period and became abundant in the Devonian period. The subsequent periods show the evolution of amphibians, reptiles, birds and mammals.
Essay # 3. General Characters of Chordata:
1. Chordates are aquatic, aerial or terrestrial. All are free-living with no finally parasitic forms.
2. Body small to large, bilaterally symmetrical and metamerically segmented.
3. A postanal tail usually projects beyond the anus at some stage of life and may or may not persist in the adult.
4. Exoskeleton often present; well-developed in most vertebrates.
5. Body wall triploblastic, i.e., presence of three germ layers- ectoderm, mesoderm and endoderm.
6. Coelomate animals, i.e., a well-developed true coelom is always present which is enterocoelic or schizocoelic in origin.
7. Notochord is always essentially present at some stage of life cycle. It is completely or partly replaced by vertebral column in majority of animals.
8. A cartilaginous or bony, living and jointed endoskeleton present in the majority of members (vertebrates).
9. Paired pharyngeal gill-slits are present on either side of the pharynx at some stage of life, may or may not be functional.
10. Digestive system is complete with digestive glands.
11. Blood vascular system is closed. Heart ventral with dorsal and ventral blood vessels. Hepatic portal system is present which is well developed.
12. Excretory system comprising proto-or meso-or metanephric kidneys.
13. Nerve cord is dorsal and tubular. Anterior end usually enlarged to form brain.
14. Sexes separate with rare exceptions.
Essay # 4. Fundamental Chordate Animals:
All the chordates possess three common fundamental characteristics at some stage in their life history.
These three common fundamental morphological features are as follows:
1. Notochord (a longitudinal supporting rod-like structure);
2. Dorsal tubular (hollow) nerve cord; and
3. Pharyngeal gill-slits.
These three distinctive characteristics are unique for the phylum Chordata (Fig. 1.1). The existence of such common characteristics is considered as a result of inheritance from a common ancestry. Besides these three common characteristics, there are few other characteristics which have less diagnostic value.
1. Notochord or Chorda Dorsalis:
The notochord (Gr., noton = back; chorda = cord) is a rod-like, elongated, elastic structure situated just above the alimentary canal and immediately beneath the dorsal tubular nerve cord. It is composed of large vacuolated notochordal cells enclosed in an outer fibrous and an inner elastic sheath of connective tissue (Fig. 1.2).
It serves as a primitive internal skeleton and acts as a rigid axis, but permits movements of the body. It may persist throughout life, as in lancelet, lamprey and some fishes, or it may be replaced partially or completely by a backbone or vertebral column. The notochord originates from the endodermal roof of the embryonic archenteron.
2. Dorsal Tubular Nerve Cord:
In chordates, there is a dorsal, hollow or tubular, fluid-filled nerve cord lying just above the antero-posterior axis of the body (Fig. 1.2). The nerve cord is formed by an infolding of mid-dorsal strip of neural ectoderm and encloses a cavity or canal, the neurocoel. The nerve cord lies above the notochord and outside the coelom, extending lengthwise in the body. This tubular nerve cord persists throughout the life in chordates.
In vertebrates, the anterior region of nerve cord becomes specialised to form a cerebral vesicle or brain which is enclosed in a protective cartilaginous or bony cranium. The posterior part of nerve cord transforms into spinal cord which is protected within the neural canal of the vertebral column.
3. Pharyngeal Gill-Slits:
The gill-slits have many alternative names, such as gill-clefts, pharyngeal or branchial-clefts, visceral-clefts, visceral or branchial-pouches. In all the chordates, at some stage of their life history, a series of paired internal gill-slits or gill-clefts perforate through the pharyngeal wall of the gut behind the mouth.
Gill-slits are paired openings leading from the pharynx to the exterior. Lateral outpushing of the endodermal lining of pharynx meet and fuse with ectodermal inpushings 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 breathe by gills, traces of gill-clefts are present during early development but disappear later in adults. Within these clefts are the gills in many aquatic animals. In many animals equipped with lungs, branchial-clefts, or branchial-grooves are always found in the embryo. In lower chordates, the visceral-clefts are used as a feeding mechanism throughout life, but in higher chordates, the gill-clefts often form some endocrine glands.
The gill-slits serve primarily for the passage of the water from the pharynx to outside, thus, bathing the gills for respiration. The water current secondarily helps in filter feeding by retaining food particles in the pharynx.
The above three common characters appear during early embryonic life of all the chordates. But all the above three characters rarely persist in the adult (e.g., Branchiostoma).
Only one of these three primary characters of chordates is found in the adults of most of them, this is the dorsal tubular nerve cord, even this has its hollow lumen greatly reduced in some, and in Urochordata the central nervous system degenerates in the adult. The other two characters completely disappear in the adults of most chordates. However, the three primary characters are possessed only by chordates.
The three common chordate characters were probably characteristics of the ancestral chordates. These characters distinguish chordates from all other animals and appear to reveal their common ancestry.
Essay # 5. Characters Common to Chordates and Higher Non Chordates:
1. Axiate Organization:
All the chordates have a distinct polar axis. The anterior end is differentiated into a cephalic region or head and the posterior end is characterised by the tail in most cases. The longitudinal axis of body running from head to tail is known as anteroposterior axis. The antero-posterior axis of the chordates corresponds to that of most of the higher non-chordates. The axiate organisation is not strictly homologous, because many fundamental differences exist between the two groups.
2. Bilateral Symmetry:
Bilateral symmetry is the similarity of the right and left sides of an animal in which any structure on one side of the median place is an image of the same organ on the other side. There is an antero-posterior axis from the head to the tail, the two sides of this axis are symmetrical.
The higher nonchordates also show bilateral symmetry, but the head end of higher chordates is not homologous with that of annelids or arthropods, because the mouth of these higher nonchordates is derived from the blastopore of the embryo, whereas in vertebrates the anus is formed near the blastopore, and the mouth is formed by an inpushing into archenteron at the opposite end.
3. Triploblastic Condition:
All Metazoa above the Coelenterata are triploblastic having three germinal layers called ectoderm, endoderm, and mesoderm, and it is assumed that the mesoderm of these groups is homologous. But in higher invertebrates the mesoderm arises as solid cord-like outgrowth from the junction of ectoderm and endoderm in gastrula stage, whereas in brachiopods, echinoderms and Branchiostoma, etc., the mesoderm is formed as lateral out- pushings of the archenteron of gastrula.
4. Coelom:
Chordata are coelomate animals; they have a true coelom lined entirely by mesoderm. There are two distinct ways in which the coelom can be formed. In lower chordata (Cephalochordata) the coelom arises as direct paired pouches or outgrowths of the archenteron which fuse in a linear series (enterocoelic coelom), in Urochordata mesoderm arises as a solid mass in the same position as outgrowths of archenteron of Cephalochordata, but they have no coelom; but in higher forms such as higher chordates and higher nonchordates.
In annelids, molluscs, arthropods and higher chordates, the coelom is formed from the lateral plate mesoderm splitting into an outer somatic and inner splanchnic layer which encloses the coelom (schizocoelic coelom).
5. Metamerism:
When certain structures of an animal which resemble each other are repeated regularly one after the other, then each such division of the body is called a metamere (segment or somite), and the animal having a series of repeated metameres is said to be metameric or to be metamerically segmented. In higher chordates metamerism is not present externally but is seen only internally in nerves, blood vessels, muscles, vertebrae, ribs and excretory system, at least in their origin.
Animals in which various metameres are almost alike are said to be homonomous in their segmentation, but this is a primitive condition and the various segments are more or less independent of each other.
In the majority of animals the metameres differ from each other in several structures, such animals are said to have heteronomous segmentation in which the segments are dissimilar and there is a division of labour among the segments, this condition is an advanced one and has arisen from the homonomous condition.
Vertebrates show only internal heteronomous segmentation and that too during development. Metamerism in chordates is not homologous with that in metameric invertebrates (annelids, arthropods) but is only analogous. 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. Metameric condition is well marked in embryonic chordates in the arrangement of muscles and in the adult vertebrates in the arrangement of vertebrae.
6. Organ-Systems:
In an organ-system, several organs work together for the same function, such as digestion, circulation and respiration, etc. Organ-system grade of organisation is exhibited by all the chordates and all the nonchordates from nemertean worms onwards. However, the vertebrates exhibit a greater stage of development and fundamental unity of organ-system grade of organisation than even the highest invertebrates.
These structural similarities shared by the chordates and the nonchordates probably reflect upon their remote common ancestry. Although it is not possible to trace exact lineage, but all available evidences indicate that chordates have evolved from the invertebrates.
Essay # 6. Subdivisions of Phylum Chordata:
Phylum Chordata is a heterogeneous assemblage of groups which differ widely from one another and exhibit various degrees of relationships to each other. In a classification of the phylum, these groups are customarily arranged in larger functional divisions or subdivisions based on specific structures or features. These subdivisions or taxa have been given different ranks under different systems of classification.
The following terminology includes major subdivisions of phylum Chordata:
1. Subphyla and Classes:
Phylum Chordata is first conveniently divided into 3 or 4 primary subdivisions called subphyla based on the character of notochord.
These are:
Subphylum 1. Hemichordata (or Adelochordata)
Subphylum 2. Urochordata (or Tunicata)
Subphylum 3. Cephalochordata
Subphylum 4. Vertebrata
Subphylum Hemichordata (Gr., hemi = half; chorde = cord) have long been considered to be the lowest chordates. But recent workers consider the so called notochord of hemichordates not a true notochord but stomochord. In recent years the hemichordates have been separated from the phylum Chordata and given the status of a separate independent invertebrate phylum.
Subphylum Urochordata (Gr.,oura = a tail; L., chorda = cord) includes 3 classes (Larvacea, Ascidiacea and Thaliacea), subphylum Cephalochordata (Gr., kephale = head; L., chorde = cord) includes a single class (Leptocardii), while the subphylum Vertebrata (L., vertebratus = backbone) is divided into 9 classes (Ostracodermi, Cyclostomata, Placodermi, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia).
2. Protochordata and Euchordata:
The first two subphyla under phylum Chordata are Urochordata and Cephalochordata. They are all marine, relatively small and without a vertebral column or backbone. They are often collectively referred to as the invertebrate chordates or nonvertebrates or protochordates (Gr., protos = first; chorde = cord). They are regarded to be early, primitive or first chordates closely allied with the ancestral chordate stock.
Earlier, the Protochordata were treated as a distinct phylum with the Hemichordata included as a third subphylum. At present, the Hemichordata is considered as an independent invertebrate phylum, while the Urochordata and Cephalochordata are considered to be true chordate subphyla. Therefore, the term “Protochordata” no longer includes any zoological taxonomic position, but is merely descriptive.
The third subphylum Vertebrata, possessing a vertebral column, is considered to be more advanced and belongs to the subdivision Euchordata of the phylum Chordata. Sometimes, the protochordates are known as the lower chordates or the euchordates as the higher chordates.
3. Acrania and Craniata:
The protochordate subphyla lack a head and cranium, therefore, they are known as Acrania (Gr., a = absent, kranion = head). On the other hand, the subphylum Vertebrata (subdivision Euchordata) possesses a distinct head and a cranium, therefore, they are known as Craniata. Typically the body of Craniata comprises a head, neck, trunk and tail.
4. Agnatha and Gnathostomata:
The Vertebrata (or Craniata) are further subdivided into two unequal sections- Agnatha and Gnathostomata. Agnatha (Gr., a = not; gnathos = jaw) lack true jaws and paired appendages. Agnatha include a small number of primitive but highly specialised fish-like forms, the extinct ostracoderms and modem cyclostomes. All other vertebrates have true jaws and paired appendages and are called Gnathostomata (Gr., gnathos = jaw; stoma = mouth). In the past Agnatha and Gnathostomata were considered as subphyla of phylum Chordata. In the recent years, they are considered as branches or groups or super classes of subphylum Vertebrata.
5. Pisces and Tetrapoda:
Basically Gnathostomata is divided into two superclasses- Pisces and Tetrapoda. The superclass Pisces (L., piscis = fish) includes all the fishes which are strictly aquatic forms with paired fins. The superclass Tetrapoda (Gr., tetra = four; podos = foot) includes four-legged land vertebrates including amphibians, reptiles, birds and mammals.
6. Anamnia and Amniota:
This method of grouping the vertebrates is based on the presence of a membrane, the amnion. The amnion holds the developing embryo in a reservoir of fluid and allows the laying of eggs on land. The animals that possess the amnion belong to Amniota which includes the classes Reptilia, Aves and Mammalia. The animals without amnion are Anamniota which includes cyclostomes, fishes and amphibians. Sometimes the anamniotes are referred to as the lower vertebrates and amniotes as higher vertebrates.