The chordates were originated from a fish-like ancestor, very similar to the larva of the Ascidia (Tunicata) and it is assumed that they became the ancestor of the chordates by retaining the larval form throughout the life. In the palaeozoic age, the chordates were originated from some sessile Echinoderms, which were bottom dwellers of the sea. The primitive chordates resemble the non-chordates in some respects and are known as protochordates or invertebrate chordates. In the evolutionary course, They acquired new novelties and became vertebrates that were radiated on all ecological niches and became strong rivals of the non-chordates in the course of time.
The phylum chordata is a very diverse phylum which contains about 43,000 living species. Among them, most organisms belong to the subphylum, Vertebrata. This phylum is considered as the third largest phylum in the animal kingdom.
Characteristics of Phylum Chordata
At different stages in their life, chordates show the following four features. They are:
- Presence of the notochord;
- Presence of the dorsal tubular nerve cord;
- Presence of the paired pharyngeal gill-slits.
- Presence of the Post-anal tail.
Notochord: It is an elastic, rod-like longitudinal structure that is made up of cartilage. It is situated immediately above the alimentary canal (digestive tract) and just below the dorsal tubular nerve cord. The notochord is formed of special type of vacuolated cells and remains covered by a sheath, known as notochordal sheath. In the invertebrate chordates (urochordata and cephalochordada), it is formed of endothermal cells, but in the case of the vertebrate-chordates, it persists in the adult (with some exception), but in vertebrates, notochord is either partially, or wholly replaced by vertebral column. The main function of notochord is to support nerve cord.
Dorsal tubular nerve cord: It is a single hollow nerve cord which consists of bundle of nerve fibers that exists dorsally along the antero-posterior axis of the body, just above the notochord. Embryologically, this nerve cord originates from the dorsal ectoderm. It connects the brain to the muscles and other organs.
Paired Pharyngeal gill-slits: A pair of openings which connect the mouth and the throat, known as Pharyngeal gill-slits. At some stages of the life, all chordates possess paired pharyngeal gill-slits. In the primary aquatic animals, these remain persist in the adult and functions as the passage for the respiratory water current through the mouth, without entering the digestive system. The transition from aquatic to the land life involves pulmonary respiration and consequently the gill-slits lose their function. However, the initial stage of their formation is clearly observed in the embryos of the terrestrial chordates. In the adults, however, these are totally obliterated and become transformed into other organs like endocrine system.
Post-anal tail: It is an extension of the body away from the anus. Some chordates bear the tail with skeletal muscles, which assist in locomotion.
The phylum Chordata also contains the following characteristic features:
Phylum Chordata is divided into the following three subphyla:
Among the above three sub-phyla, the major subphylum is vertebrata because it contains a variety of organisms like fishes, amphibians, birds, reptiles and mammals while the first two sub-phyla, urochordata and cephalochordata are collectively known as protochoradata that have very few species in between them which are all marine animals. Besides, the phylum vertebrata bears a distinguishing backbone that is built up of bone or cartilage. They have a brain which is enclosed in a skull. They also have proper nervous system, circulatory system, and a skeletal system that provides proper shape and support.
The main characteristics of urochordata are:
Examples: Ascidia mentula , Salpa fusiformis, Doliolum nationalis.
The sub-phylum Urochordata contains the following three classes:
Example: Ascidia mentula
Example: Doliolum denticulatum
Example: Oikopleura longicaudata
Sub-phulum: Cephalochordata (Acraniata)
Examples: Mediterranean amphioxus or European lancelet: Branchiostoma lanceolatum
Sub-phylum: Vertebrata (Craniata)
The sub-phylum vertebrata incorporates the remarkable chordates which occupy high taxonomical ranks. All the vertebrates are placed under the single sub-phylum Vertebrata or craniata. The vertebrate possess a ‘back-bone’ which is the most characteristic features of the vertebrates. Thus, a vertebrate is defined as an animal having back bone consisting of a series of small bones (ring-like)or cartilage running throughout the length of the body along the mid-dorsal side, enclosing and protecting the spinal cord. On the other hand, all animals which do not possess vertebral column, are known as invertebrate. The vertebrates have acquired a number of characteristics features in addition to the basic chordates characterizes which are of high adaptive value and as much they are readily distinguished from the invertebrate-chordates.
Characteristics of sub phylum Vertebrata
Sub Phylum Vertebrata is further classified into the following classes. They are:
Examples: Petromyzon marinus (Lamprey) and Myxine glutinosa (Atlantic Hagfish).
All bony fishes belong to the class Osteichthyes. All the representatives of Osteichthyes are cold-blooded which bear one pair of gill openings for breathing and different types of fins for swimming. Besides these, they have bony skeleton, jaws and one pair nostrils. The class Osteichthyes is the largest group of vertebrates in the world and it contains more than 30,000 fish species which account for about 96% of all fish species. This class does not contain sharks, rays, skates, lampreys and hagfishes. They have very keen sense organs with very good eyesight. They take a wide variety food, among them, some are herbivores, some carnivores and some take any kind of food at all(omnivores).
The class Osteichthyes is divided into the following two subclasses:
Examples: Labeo rohita, Catla catla, Anabas testudineus, Lates calcarifer, Tenualosa ilisha, Ompok pabo, Heteropneustes fosilis
The term ‘amphibian’ is derived from the Greek word ‘amphíbios’ meaning both kinds of life. They are cold-blooded vertebrate tetrapods which inhabit a wide variety of habitats, including terrestrial, arboreal, fossorial, or freshwater aquatic ecosystems. There are approximately 8,100 species, of which, about 90% are frogs. Paedophryne amauensis is the world smallest amphibians, found in Papua New Guinea which grows up to 7.7 mm in length while the largest living amphibian is Andrias sligoi (South China giant salamander) which grows up to 1.8 m in length (5 ft 11 in).
General Characteristics of Class Amphibia
You might also read: Detailed Classification of Amphibia
Class –6: Reptilia
The word reptile comes from Latin “Reptilis’ meaning creeping. They are the creeping and burrowing vertebrates with epidermal scales. There are about 8,000 living reptiles species in the world today that inhabit all continents on Earth except Antarctica. It includes verities group ectothermic vertebrates including lizards, snakes, crocodiles, alligators, turtles, caimans and worm-lizards, etc. They can inhabit different type habitats such as forests, deserts, freshwater wetlands and the open seas.
Examples: Green sea turtle (Chelonia mydas), Aldabran giant tortoise ( Aldabrachelys gigantea ), Spectral pygmy chameleon (Rhampholeon spectrum), Oriental Garden lizard (Calotes versicolor), Saltwater crocodile (Crocodylus porosus), Naja naja (Cobra), Indian Krait ( Bangarus caeruleus),Horned viper ( Vipera ammodytes).
You might also read: Detailed Classification of Extant Reptiles
Class –6: Aves
All birds belong the class Aves, they are egg-laying endothermic vertebrate animals. There are about 10,000 living species of birds in the world that inhabit in nearly all habitats on Earth. It constitutes the largest number of species among the tetrapod classes.
Examples : Corvus splendens ( house crow), Columba livia (Domestic pigeon), Indian vulture (Gyps indicus) etc.
You might also read: Classification of Aves in details
The word ‘Mammalia’ is derived from Latin word ‘mamma’ meaning "breast". The class Mammalia includes the vertebrate animals which contain mammery glands. These glands are active in female and produce milk for feeding their babies. These vertebrate animals are also characterized by the presence of fur or hair, a four-chambered heart and three middle year bones which distinguish them from birds and reptiles. There are about 5500 living mammal species under 161 families and 29 orders. The rodents, bats and shrews constitute the largest order which is followed by the Primates, Cetartiodactyla and Carnivora, respectively.
Examples: Oviparous – Platypus (Ornithorhynchus anatinus); Viviparous – Royal Bengal Tiger (Panthera tigris),Red kangaroo (Macropus rufus), flying fox (Pteropus vampyrus), Short-beaked common dolphin (Delphinus delphis) , blue whale (Balaenoptera musculus) , etc.
Difference Between Chordates and Non-chordates
They are cold or worm blooded organisms.
They are cold blooded organisms.
Notochord is present at some stage or it is replaced by a ring like vertebrae at adult stage.
Notochord or backbone is absent.
They have central nervous system which is dorsal, hollow and single.
In this case, central nervous system is ventral, solid and double.
Pharynx is perforated by gill-slits.
In this case, gill-slits are absent.
They perform respiration through gills or lungs.
They perform respiration through body surface, gills,or tracheae.
The position of heart is ventral.
Usually, the heart is absent, if present the position of heart is dorsal or lateral.
A post-anal part or tail is present.
In this case, post-anal tail is absent.
They are true coelomate organisms.
They may be acoelmate, pseudocoelmate or truly coelmate organisms.
Blood vascular system is closed type.
Generally, blood vascular system is absent; if present, it may be open or closed type.
Their body is bilaterally symmetrical.
Their body is radial, bi radial, bilateral or without symmetry.
Nerve cord is single, dorsal, without ganglia.
Nerve cord is double, ventral, usually with ganglia.
The gut is situated ventral position to nerve cord.
In this case, gut is found dorsal position to nerve cord.
They mainly perform sexual reproduction.Enter your text here...
In this case, asexual reproduction is predominant.
Their regeneration power is usually poor.
Their regeneration power is usually good.
They possess both exoskeleton and endoskeleton
In this case, only exoskeleton is present.
Examples: Hemichordates, Cyclostomes, Aves, Reptiles, Amphibians, Mammals , etc.
Examples: Protozoans, Annelids, Arthropods, Echinoderms, etc
Chordates are animals which belong to the phylum Chordata. This phylum consists of three groups, namely vertebrates, tunicates, and lancelets. The members of chordates are very essential for an ecosystem because they keep up the ecosystem. Chordates consist of 43,000 living species which can consume and hunt down other types of animals and maintain of predator and prey role. Many chordates provide food for human beings and many chordates release their excretion to the environment, provide nutrients and minerals to the ecosystem which boost up the plants growth.
You might also read: Chondrichthyes Vs Osteichthyes:General Characteristics and Differences