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In the scheme of classification, the animals kingdom is divided first into several majore animal groups called phyla. There are approximately 30 animal phyla currently recognized. The last major group of the animal kingdom is known as phylum chordata. It was created by Balfour in 1880. The name of this phylum is derived from two Greek words, the chorde means a string or cord, and ata means bearing. Thus, chordates are animals having a cord, i.e., notochord. The animals belonging to all other phyla of the animal kingdom are often termed 'the non-chordates' or 'the invertebrates' since they have no notochord or backbone in their body structure.
Diversity of chordates:
The chordates show an astonishing diversity of form, physiology and habitat.
Numerical strength: The number of chordate species is not unusually large. About 49,000 species are on record which are only half of the living species of mollusks, and less than 1/10 those of arthropods. The two subphyla Urochordata and cephalochordate claim for nearly 2,500 species. The subphylum vertebrata includes 46,500 species of these; fishes are the most numerous with an approximately 25,000 species. It is commonly assumed that amphibian species number about 2,500, reptiles 6,000, birds 9,000 and mammals 4,500.
Size: Despite their modest number of species, the chordates make a disproportionate contribution to the biomass of the earth. Nearly all of them are medium to large in size. The vertebrates, in particular, are considerably large and many of them are among the largest of living animals. The gigantic blue whale (Balaenoptera musculus) and the smallest fish Philippine goby (Pandaka).
Ecology: The chordates are not only the largest animals in existence today, but ecologically they are among the most successful in the animal kingdom. They are able to occupy most kinds of habitats and they have adapted themselves to more modes of existence than any other group, including the arthropods. They are found in the sea, in fresh water, in the air, and on all parts of land from the poles to the equator. Birds and mammals have been able to penetrate cold climate because they have a constant body temperature, something no other animals have.
Three fundamental chordate characters:
Dorsal hollow nerve cord
The central nervous system of the chordates is present dorsally in the body. It is in the form of a longitudinal, hollow or tubular nerve cord lying just above the notochord and extending lengthwise in the body. The nerve cord or neural tube is derived from the dorsal ectodermal neural plate of the embryo and encloses a cavity or canal called neurocoel. There are no distinct ganglionic enlargements. The nerve cord serves for the integration and coordination of the body activities.
Notochord or chorda dorsalis
The notochord is an elongated rod-like flexible structure extending the length of the body. It is present immediately beneath the nerve cord and just above the digestive canal. It originates from the endodermal roof of the embryonic archenteron. Structurally, it is composed of large vacuolated notochordal cells containing a gelatinous matrix and surrounded by an outer fibrous and an inner elastic sheath.
Pharyngeal gill slits
In all the chordates, at some stage of their life history, a series of paired lateral gill clefts or gill slits perforate through the pharyngeal wall of the gut behind the mouth. These are variously termed as pharyngeal, branchial and visceral clefts or pouches. They serve primarily for the passage of water from the pharynx to outside, thus bathing the gills for respiration. The water current secondarily aids in inter feeding by retaining food particles in the pharynx.
The three common chordates characters were probably characteristics of the ancestral chordates. They distinguish chordates from all other animals and appear their common ancestry.
Characters common to chordates and higher non-chordates:
There are a number of features in which the chordates resemble the higher nonchordates or invertebrates.
AXIATION: The body in both has a distinct polar axis. The anterior end differentiated into a cephalic region or head that usually proceeds in locomotion. The opposite posterior end forms a tail in most cases. This longitudinal axis of the body running from head to tail is known as the antero-posterior axis.
BILATERAL SYMMETRY: Due to existence of longitudinal antero-posterior axis, the body of all chordates and most higher nonchordates exhibits bilateral symmetry i.e.; the right and left sides of the body are the exact mirror image of each other.
COELOM: A secondary body cavity or true coelom exists between the body wall and the digestive tube, and it is lined on all sides by mesoderm. However, it differs in its mode of origin in different groups of chordates and nonchordates.
TRIPLOBLASTIC CONDITIONS: Invertebrates above the level of coelenterates and all chordates are triploblastic animals. They have three germ layers: - ectoderm, endoderm, & mesoderm.
METAMERISM: Metamerism is a condition in which the body is composed of a linear series of similar body segments, called metameres or somites. It is found in three phyla: Annelida, Arthropoda and Chordata. In Annelid and Arthropoda, metameric segmentation is both internal as well as external, whereas in chordates it is less clear external.
ORGAN SYSTEMS: In an organsystem, several organs work together for the same function, such as digestion, circulation, respiration, etc. It is shown by all chordates and all the non chordates from nemetean worms onwards. However, the vertebrates show a greater stage of development and fundamental unity in this than even highest invertebrates.
Advancement of Chordata over other phyla:
Phylum chordate have some advantages over other phyla due to certain characters.
Living endoskeleton: with the exception of echinoderms and a few others, only chordates possess a living endoskeleton. It grows in size with the rest of the body so that there is no compulsion to shed it periodically to allow for growth like the non living chitinous exoskeleton of the nonbchordates phyla. Further, this living endoskeleton permits greater freedom of movement and indefinite growth so that many chordates are the largest creatures of animal world.
Efficient respiration: the gills in aquatic chordates and the lungs in terrestrial forms form efficient organs of respiration. The tracheal system of certain arthropods is also efficient but is suitable to animals of small size only.
Efficient circulation: the circulatory system of chordates is well developed and the blood flows freely in the respiratory organs ensuring rapid exchange of gases. Moreover, the blood-vasular system forms an important medium for several other vital activities of the body.
Centralized nervous system: the invertebrate phyla show a growing tendency of centralization of nervous system, reaching its culmination in the higher chordates. The sensory systems explains the great power of the chordates for adapting themselves most successfully to a variety of environments.
Origin and ancestry of chordate:
While a great deal is known about modern chordates, including the lower forms, their origin remains obscure. Scientists have not succeeded in determining which lower forms have given rise to them. Their early ancestors most likely were soft-bodied and left no definite fossils remains. They must have originated prior to Cambrian period as the oldest fossils of known vertebrates have been discovered in late Cambrian strata. Most scientists consider that the chordates have originated from invertebrates. Several theories attempt to explain the origin of chordates from nonchordate groups, but they have serious drawbacks and are far from being satisfactory. One theory advocates the descent of Chordata from the Echinodermata as such. The remarkable similarities between the echinoderm and hemichordate larvae are taken as good evidence for common ancestry. Garstang suggested that probably free-swimming auricularian larvae of some ancestral echinoderms evolved in to chordates through paedogenesis, i.e., prolongation of larval life without undergoing metamorphosis and reproducing sexually. Most zoologists now favor the deuterostome line of chordate evolution, according to which the phyla Ehinodermata, hemichordate, & chordate 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 late Cambrian and Ordovician. A no. of fishes followed in Silurian and became abundant in the Devonian. The subsequent periods show the evolution of amphibians, reptiles, birds, & mammals.
General characters of phylum chordates:
Aquatic, aerial, or terrestrial. All free-living with no fully parasitic forms.
Body small to large, bilaterally symmetrical and metamerically segmented.
A postanal tail usually projects beyond the anus at some stage and may or may not persist in the adult.
Exoskeleton often present; well developed in most vertebrates.
Body wall triploblastic with three germinal layers: ectoderm, mesoderm, & endoderm.
Coelomate animals having a true coelom, enterocoelic schizocoelic in origin.
A skeletal rod, the notochord, present at some stage in life cycle.
A cartilaginous or bony, living & jointed endoskeleton present in the majority of members.
Pharyngeal gill slits present at some stage; may or may not be functional.
Digestive system complete with digestive glands.
Blood vascular system closed. Heart ventral with dorsal and ventral blood vessels. Hepatic portal system well developed.
Excretory system comprising proto-or meso-or meta-nephric kidneys.
Nerve cord dorsal and tubular. Anterior and usually enlarged to form brain.
Sexes separate with rare exceptions.
Comparison of chordates with nonchordates:
Radial, biradial, bilateral or lacking
True or pseudo Metamerism or lacking
Grade of organization
Protoplasmic to organ-system
Diploblastic or Triploblastic or absent
Acoelomate, pseudocoelomate or truly coelomate
From several segments
From same segment
Present at some stage or replaced by a backbone made of ring like vertebrae
Notochord or backbone lacking
Ventral to nerve cord
Dorsal to nerve cord
Present at some stage of life
Differentiated and opens before the last segment
Opens on the last segment or absent
Blood vascular system
Open, closed or absent
Dorsal, lateral or absent
Dorsal blood vessels
Blood flows posteriorly
Blood flows anteriorly
Hepatic portal system
In red corpuscles
In plasma or absent
Through gills or lungs
Through body surface, gills or tracheae
Dorsal to pharynx in head
Above pharynx or absent
Single, dorsal, without ganglia
Double, ventral, usually bearing ganglia
Roots of segmental nerves
Dorsal and ventral separate
Dorsal and ventral roots not separate
Sexual reproduction predominant
Asexual reproduction predominant
Cold or warm-blooded
Major subdivisions of phylum chordata:
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Brief classification of chordate with characters:
The chordates form a large heterogeneous grouping of members differing widely from one another in many respects. This creates problems in their systematic classification. Different schemes have been proposed by a number of taxonomists from time to time. The one followed in this is a synthesis of the most recent ones adopted here for the sake simplicity and proper understanding
Phylum chordate can be divided in to two groups; Acrania and Craniata having contrasting characters.
Group 1 ACRANIA (protochordata)
All marine, small, primitive or lower chordates. Lacking a head, a skull or cranium, vertebral column, jaws and brain. About 2000 species. The Acrania is divided in to three sub phyla: hemichordate, urochordata, and cephalochordate chiefly on the position of notochord.
Subphylum 1: HEMICHORDATA
Body divided in to three regions: proboscis, collar and trunk. Notochord doubtful, short, confined to proboscis and non homologous with that of chordates.
Class 1: Enteropneusta Body large and worm-like. Gill-slits numerous. Intestine straight. Acorn or tongue worms. 3 families, 15 genera and 70 species. Balanoglossus, saccoglossus.
Class 2: Pterobranchia Body small and compact. Gill slits one pair or none, intestine U-shaped. Pterobranchs includes 2 orders, 3 genera and 20 species. Cepalodiscus, rhabdopleura.
Class 3 Planctosphaeroidea Transparent, round and specialized tornaria larva, having extensively branched ciliary bands and L-shaped alimentary canal, represents this class. Planctosphaera pelagica.
Class 4: Graptolita The fossils graptolites were abundant in Ordovician and Silurian periods and often placed as an extinct class under hemichordate. Their tubular chitinous skeleton and colonial habits show an affinity with Rhabdopleura. Dendrograptus.
Subphylum 2: UROCHORDATA or TUNICATA
Notochord and nerve cord only in tadpole-like larva. Adult sac-like, often sessile and encased in a protective tunic. Tunicates.
Class 1: Ascidiacea Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-clefts numerous. Ascidians or sea squirts. 2 subclass, 3 orders, 12 families, 37 genera and 1200 species. Herdmania, Ciona, Molgula.
Class 2: Thaliacea Free-swimming or pelagic tunicates with circular muscles in tunic. Sometimes colonial. Salps or chain tunicates. 3 orders, 5 families, 9 genera and 30 species. Salpa, Doliolum, Pyrosoma.
Class 3: Larvacea of Appendicularia Tiny, transparent, free-floating. Adults retain many larval features including tail. Only two gill-slits. 2 orders, 2 families,5 genera and 30 species. Oikopleura.
Subphylum 3: CEPHALOCHORDATA
Notochord and nerve cord present throughout life along entire length of body.
Class leptocardii Body fish like, segmented with distinct myotomes and numerous gill-slits. Free-swimming and burrowing. Lancelets. One class, one family, 2 genera and 30 species. Branchiostoma, Asymmetron.
Group 2 CRANIATA (Euchordata)
Aquatic or terrestrial, usually large-sized, higher chordates or vertebrates with distinct head, a vertebral column, jaws and brain protected by a skull or cranium. The Craniata includes a single subphylum, the vertebrata.
Subphylum 4: VERTEBRATA
Notochord supplemented or replaced by a vertebral column or backbone composed of overlapping vertebrae. Body divisible into head, neck, trunk and tail. Usually dioecious. This subphylum is divided into two divisions:
Division 1: Agnatha
Jawless primitive fish like vertebrates without true jaws and paired limbs.
Class 1: Ostracodermi Several extinct orders of ancient primitive heavily armoured, palaeozoic, world's first vertebrates, collectively called the ostracoderms. Cephalaspis, Drepanaspis.
Class 2: Cyclostomata Body eel-shaped, without scales, jaws and lateral fins. Mouth rounded and suctorial. Gills 5-16 pairs. Parasites and scavengers. 45 species. Lampreys (Petromyzon) and hag fishes (Myxine).
Division 2: Gnathostomata
Jawed vertebrates having true jaws and paired limbs. For convenience, some taxonomists further divided gnathostomata into two superclasses. All the fishes and fish like aquatic gnathostomes are placed in the superclass Pisces, whereas all the four-footed terrestrial gnathostomes in the superclass Tetrapoda. Their contrasting features are as follows:
Superclass 1: PISCES
Fishes or fish-like aquatic forms with paired as well as median fins, gills and scaly skin
Class 1: Placodermi Several extinct orders of primitive earliest jawed fishes of palaeozoic with bony head shield movably articulated with trunk shield. Placoderms, Climatius, Dinichthys.
Class 2: Chondrichthyes Mostly marine. Cartilaginous endoskeleton. Skin with placoid scales. Gill-slits not covered by operculum. Pelvic claspers in male. Cartilaginous fishes. Approximately 600 species. Scolidion (dog fish), Chimaera (rat fish)
Class 3: Osteichthyes Freshwater and marine. Endoskeleton mostly bony.skin having various types of scales other than placoid. Gill-slits covered byan operculum. Males without claspers. 20,000 species. Labeo (rohu), Protopterus (lung fish), Hippocampus (sea horse).
Superclass 2: Tetrapoda
land vertebrates with two pairs of pentadactyle limbs, cornified skin and lungs.
Class 1: Amphibia Larval stage usually aquatic and breaths by gills. Adult typically terrestrial and respires by lungs. Skin moist, glandular and with no external scales. Heart 3-chambered. Approximately 2500 species. Rana (frog), Bufo (toad), Ambystoma (salamander).
Class 2: Reptilia Terrestrial tetrapods, skin dry, covered by ectodermal horny scales or bony plates. Heart incompletely 4-chambered, Cold-blooded, respiration by lungs, 7000 species. Hemidactylus (wall lizard), Uromastix (spiny-tailed lizard), Naja (cobra), Sphenodon, Crocodilus.
Class 3: Aves Typically flying vertebrates covered with feathers. Fore limbs modified in to wings. No teeth in beak. Heart 4-chambered. Warm blooded. About 9000 species. Struthio (African ostrich), Columba (pigeon), Gallus (fowl).
Class 4: Mammalia Body covered by hair. Skin glandular. Female with mammary glands which secrete milk for suckling the young. Heart 4-chambered. Warm blooded. Air breathing vertebrates. 4500 species. Echidna (spiny anteater), Macropus (kangaroo), Rattus (rat), Homo (man).
Chordate zoology by E.L.Jordan and P.S.Verma
Modern textbook of zoology, vertebrates (animal diversity-2) by R.L.Kotpal