In this article we will discuss about the meaning and classification of chordata.
Meaning of Chordata:
The Chordata (chorde = cord) includes well-known animals such as lampreys, fishes, amphibians, reptiles, birds and mammals. They also include protochordates such as balanoglossus, ascidia and amphioxus. All chordates possess a notochord at least for some time in their life-history.
It is the notochord that has given the name chordate to this group. The notochord is an elastic supporting rod covered by one or two sheaths of connective tissue and is composed of large vacuolated cells. It is situated in the mid-dorsal line and typically extends along the whole length of the animal, being placed under the nerve cord but above the alimentary canal.
It persists throughout life in the lower forms, such as amphioxus and lamprey. In the adults of higher chordates, the notochord is replaced, either wholly or partially, by a skeletal structure known as the vertebral column. Such chordates are the vertebrates.
We are already familiar with the structure of one vertebrate animal, the toad, which possesses a distinct vertebral column. But back-bone is not all. There are other essential features by which the vertebrates like toad, may be distinguished from the invertebrates such as earthworm, leech and prawn. These are now presented in a tabular form.
Classification of Chordata:
Subphylum I. Protochordata (protos = first), line chordates with no true brain or skull.
Hemichorda (hemi— half) or Adelochorda (adelos = concealed; chorde—cord). Marine, worm-like chordates with pharyngeal gill-slits and a stiff rod, suggesting notochord, at the anterior region of the body. Balanoglossus, Dolichoglossus, etc.
Urochorda (oura= tail; chorde —cord) or Tunicata (tunica = envelop). Marine sac-like animals in which the adults are sedentary (except in one group) and lost most of the chordate characteristics, but the free-swimming larvae possess (i) a notochord in its tail (hence urochorda), (ii) pharyngeal clefts, and (iii) a dorsally placed hollow nerve tube. During metamorphosis the tail is lost and the notochord along with the dorsal nerve tube is thus sacrificed. Salpa, Ascidia or sea squirt, Botryllus, etc.
Cephalochordate (Kephale-head; chorde —cord) or Acrania. Marine, fish-like creatures with a notochord extending from the tip of the tail to the tip of the snout (hence cephalochordate). Amphioxus (Branchiostoma) or the lancelets.
Subphylum. II. Vertebrata:
(Vertebra =a joint) or CRA- NIATA (Kranion = skull). Higher chordates with a definite brain enclosed in the skull (hence craniata). Notochord is replaced in the higher forms by a metameric vertebral column. There is a living endoskeleton composed of bones and cartilages, a true coelom, and a hollow tubular central nervous system placed in the mid-dorsal line. Lampreys, fishes, amphibians, reptiles, birds and mammals.
There are two branches of the vertebrates:
(i) The round- mouthed Agnatha in which teeth and jaws are absent, and
(ii) The Gnathostomata in which the mouth is surrounded by upper and lower jaws. The cyclostomata are the living agnatha and the rest are gnathostomata.
Class I Cyclostomata (Kyklos = Circle; Stoma = Mouth):
Primitive marine vertebrates with round, suctorial mouth and rasping tongue bearing horny epidermal teeth. Notochord persists throughout life. Endoskeleton is poorly developed and cartilaginous. Paired fins and girdles are absent. Skin is slimy and without any scales. The modern lampreys (Petromyzon) and the hags (Myxine) are the two living representatives.
Class II Chondrichthyes (Chondros = Cartilage; icthys—fish):
This class includes sharks, rays, etc. The most striking character of fishes of this class is its entirely cartilaginous skeleton. No true bone is present. An operculum is absent (except in Holocephali—a group of chondrichthyes).
There are five gill-slits, on each side, which open directly to the exterior. The body is covered with placoid scales, which are provided with minute spines at the base. The cloaca receives the rectum, excretory and reproductive ducts. Several species are viviparous.
Scoliodon (Indian shark), Narcine (electric ray), saw fishes, etc.
Class III Osteichthyes or Actinoptergh:
This class includes most of the common fishes. Bony skeleton is present throughout the class. An operculum on each side covers the filamentous gills. The fins are supported by fin-rays.
Bhetki, Ruhu, Sole, Lata, etc.
Class IV Choanichthyes:
Dipnoi, one of the existing groups of this class, possess distinct lungs and are of great evolutionary significance in giving rise to land vertebrates.
Epiceratodus or Ceratodus (found in Australia). Lepidosiren (found in South America), and Protopterus (found in Africa).
The Latimeria chalumnae, which is the only living member of a nearly extinct side branch, the Crossopterygii, of the class. Choanichthyes, was found recently near South Africa. These three existing fish groups, namely Chondrichthyes.
Actinopterygii and Choanichthyes along with an extinct group, the Aphetohyoidea, are placed under the super-class Pisces (pisces= fishes). The entire super-class represents the largest stock of vertebrates, who have dominated the water of the earth.
The following are the characteristics of the fishes in general:
(i) ‘Cold-blooded’ vertebrates breathing by means of gills (except in Dipnoi, where lung respiration is also found).
(ii) They have a streamlined body in accordance with the aquatic mode of life.
(iii) Skin is usually covered by scales.
(iv) Pectoral and pelvic fins are usually paired, and there are unpaired median fins. All the fins are supported by fin-rays.
(v) There is no tympanic membrane.
(vi) The heart consists of sinus venosus, one auricle and one ventricle. It contains only deoxygenated blood (except in Dipnoi, where the auricle shows indication of partition and also receives oxygenated blood).
Class V Amphibia (Amphi = Both; Bios = Life):
The skin of amphibia is moist due to secretion of mucus and serves as a huge respiratory surface. Scales are usually lacking. As the name implies, they lead a double life. The larva is always aquatic and breathes by gills, whereas the adults are lung-breathing terrestrial creatures with two pairs of limbs.
(i) The tailed caudata (cauda = tail) or urodela (oara = tail; delos = visible) such as Necturus (Mud-puppy) and Salamandra;
(ii) the tailless Anura or Salientia (salio = leap) such as frogs and toads, and
(iii) the limbless Apoda (A =no; podus = foot) such as the Coecilian Icthyophis. In some of the tailed amphibians gills persist throughout life (Proteus; Necturus). Fins, when present, are never supported by fin-rays.
Class VI. Reptilia:
The reptiles are craniates with complete adaptation to life on dry land. They have dry skin with epidermal scales. Lungs are the sole respiratory organs even in aquatic reptiles such a)s tortoises and crocodiles.
The development is direct and a larval stage is absent.
The modern reptiles consist of the following four orders:
(i) Chelonia (ckelone—tortoise) including tortoises and turtles,
(ii) Rhynchocephalia including the New Zealand lizard Sphenodon;
(ii) Crocodilia including gavials, crocodiles and alligators;
(iv) Squamata including the snakes, lizards and chameleons.
Class VII. Aves (Am = Bird):
Birds are provided with a covering of feathers and the forelimbs are modified to form the wings. The jaws are toothless in all modern birds.
The living birds are divided into two groups:
(i) Ratitae including the flightless running forms, such as Ostrich, Emu and Kiwi;
(ii) Carinatae or the flying birds, such as pigeons, parrots, crows.
Class VIII. Mammalia (Mamma = Breast):
Mammals are the highest of the vertebrates and include man. The skin is covered by a coat of hair though this may be reduced to a few bristles as in the whales. The young are nourished by milk, a secretion from the breast-gland of their mother.
The mammals comprise three subdivisions:
(i) The egg-laying Prototheria (protos= first; therion= mammal) including the duck-billed mole, Ornithorhynchus and the spiny ant-eater, Echidna;
(ii) The pouched Metatheria (meta — beyond; therion—mammal) including the marsupials such as Kangaroos;
(iii) Eutheria (Eu= true; therion— mammal) including the true placental mammals whose embryo remain attached by means of a structure called placenta, to the womb of their mother. Respiration and nutrition of the embryo is carried on through the placenta.
Examples:—Moles, bats, sloths, armadillos, pangolins, beavers, rats, cats, seals, walruses, horse, elephants, rhinoceros, sea- cows, whales, dolphins, lemurs, monkeys, apes and man.