Chordata are highly developed and evolved organism that belongs to animal kingdom.
Specific Characteristics of Chordata
The members of Phylum-chordate (L. Chorda = string or cord) possess three distinctive characters at some stages in their life history.
Those distinctive characteristics are as follows:
- A longitudinal rod-like notochord.
- A dorsal tubular nerve cord.
- A series of pharyngeal gill slits.
- Notochord: The notochord is a rod-like structure made up of tough vacuolated cells, surrounded by a fibrous sheath. It is a stiff, elastic, longitudinal rod found between the nerve cord and alimentary canal forming an axial skeleton.
Cephalochordata: Notochord is permanent throughout life.
Fishes, amphibians, reptiles, birds and mammals – notochord replaced by the vertebral column. The notochord renders support to the body and provides a surface for the attachment of muscles.
2. Nerve Cord: A dorsal, hollow and tubular nerve cord running throughout the body is present in chordates. It is derived from ectoderm. It expands in the head region and forms the brain.
Amphioxus- A cerebral vesicle is present in the adult. Fishes, frogs, reptiles, birds and mammals-a brain and spinal cord are present.
3. Pharyngeal gill slits: There is a series of lateral symmetrically arranged, paired apertures in the pharyngeal wall leading from the pharynx to the exterior. There are known as pharyngeal clefts or gill slits or branchial clefts.
The Gill slits are numerous in protochordate and serve as food gathering devices, but in reptiles, birds and mammals gill slits are modified differently.
Gill slits are mainly respiratory in fishes, in reptiles, birds and mammals gill slits are modified differently. Gill slits are mainly respiratory in fishes.
General Characteristics of Chordata
- Chordates live in aquatic, aerial or terrestrial habitats.
- Body bilaterally symmetrical and metamerically segmented.
- Body triploblastic, i.e., with three germinal layers-ectoderm, mesoderm and endoderm.
- Coelomate animals having a true coelom.
- The nerve cord is dorsal and tubular. Its anterior end is usually enlarged, forming the brain.
- An elastic longitudinal rod called the notochord is present at some stage in the life cycle. In vertebrates it is replaced by the vertebral column.
- Pharyngeal gill slits are present at some stage of the life cycle. They may or may not be functional.
- Exoskeleton usually present.
- Endoskeleton cartilaginous or bony, present in most of the members (vertebrates).
- The digestive system is complete and with digestive glands.
- Blood vascular system of closed type. Heart ventral in position. Hepatic portal system well development.
- The excretory system consists of proto or meso or meta-nephric kidneys.
- The reproductive system is well developed. Sexes are separate with rare exceptions.
- The post-anal tail is present.
- Examples of Chordates: Amphioxus, Herdmania, Balanoglossus
Classification of Phylum Chordata
This phylum includes about 100,000 known species. Concerning the detailed classification of the phylum chordate, there are differences of opinion among zoologists. Here the following classification has been presented.
Phylum Chordata has been divided into 4 sub-phyla based on the extent of notochord.
- Sub-phylum: Hemichordate
- Sub-phylum: Tunicate or Urochordate
- Sub-phylum: Cephalochordate
- Sub-phylum: Vertebrata
The sub-phyla Hemichordate, Urochordate and Cephalochordate are treated as Proto-chordates or Acrania (Gk. A: absent, cranium-skull). Nowadays some Zoologists consider Hemichordate under non-chordate.
1. Sub-phylum: Hemichordata
Gk: Hemi-half, chorde-cord
Characters of Hemichordates
- Marine animals living on the sandy bottom.
- Worm-like, soft body, divisible into anterior proboscis, median collar and elongated posterior trunk.
- Gill slits permanent
- Example: Balanoglossus
2. Sub-phylum: Urochordata
Gk: Oura-tail, chorde-cord
Characteristics of Urochordate
- A marine animal that is sedentary in habit.
- Saclike body, enclosed in a cuticular runic or test.
- Presence of notochord in the tail region of the larval tadpole but absent in the adult.
- Example: Herdmania (Sea-squirts)Salpa, Doliolum
3. Sub-phylum: Cephalochordata
Gk. Kephale-head, chorde-cord
Characteristics of Cephalochordata:
- Marine animals.
- Body fish like, elongated, laterally compressed and pointed at both ends.
- Presence of segmented muscles (myotomes).
- Notochord extends the entire length of the body and persists throughout life.
- Example: Lancelet (Amphioxus)
4. Sub-phylum: Vertebrata
- Skull with the cranium (braincase) and presence of visceral arches.
- Presence of vertebral column which replaces notochord.
This sub-phylum vertebrata (also called craniata) compromises about 40,000 species and divided into two sub-divisions.
a. Sub-division: Agnatha, b. Sub-division-Gnathostomata
- Without jaws.
- No paired fins (appendages).
This sub-division Agnatha comprises only one living group of animals arranged under the class Cyclostomata.
Characterstics of Cyclostomata
G.k. Cyklos = circular, stoma = mouth
- The freshwater of marine.
- Cold-blooded, fish-like vertebrates.
- Presence of a circular and suctorial mouth provided with horny teeth.
- No true jaws.
- Scales and lateral fins absent but only median fins present.
- Body rounded, slender and eel-like.
- Single nasal sac.
- Example: Lamprey (Petromyzon), Hagfish (Myxine)
Gk. Gnatha: Jaw, stoma: mouth
Characteristics of Gnathostoma
- True jaws are present.
- Usually paired appendages are present.
The subdivision Gnathostomata consists of two super classes as follows:
- Superclass- Tetrapoda
Super class – Pisces
Characteristics of Pisces
- Aquatic (freshwater or marine).
- The body is covered with an exoskeleton of dermal scales.
- Presence of paired appendages in the form of pectoral and pelvic fins.
- Respiration occurs by gills lining the pharyngeal clefts.
The super-class Pisces is sub-divided into four classes namely:
- Elasmobranchii (cartilaginous fishes),
- Holo cephali (chimaera)
- Dipnoi (lung fishes) and
- Teleostomi (bony fishes).
Here only two classes has been described, necessary for the beginners.
Characteristics of Elasmobranchil
- Includes cartilaginous fish and mostly marine in habit.
- Skeleton is cartilaginous in nature.
- The notochord is persistent in the adult.
- Skin is covered with placoid scales.
- The mouth is ventral in position.
- No operculum.
- 5 to 7 pairs of gill clefts are found.
- Examples: Shark (Scoliodon), Electric ray (Torpedo). Common ray (Raja), Hammer headed shark (Zygaena).
The class Elasmobranchii includes about 550 known species.
Characteristics of Teleostomi
- Includes bony fishes inhabiting fresh water as well as marine.
- Skeleton is bony in nature.
- Four pairs of gills, covered by operculum.
- Skin is covered by cycloid, ctenoid or ganoid scales absent in few cases.
- The mouth is terminal.
- Usually, an air bladder is present.
- Examples: Catla, Rohu (Labeo), Anabas, Stickle back.
The class Teleostomi includes about 30,000 known species.
**According to a recent view, the terms Elasn obranchii and Teleostomi have been used in this text instead of Chondrichthyes and Osteichthyes respectively.
(Gk. Tetra-four, Podos-foot)
Characteristics of Tetrapoda
- Mostly terrestrial (land vertebrates).
- Presence of paired appendages in the form of pentadactyle (five digited) limbs.
- Cornified skin is present.
- Respiration is through the lungs.
The super-class Tetrapoda is sub-divided into four classes: namely: (i) Amphibia (ii) Rep:ilia (iii) Aves and (iv) Mammalia.
(Gk. amphi-both, bios-life).
Characteristics of Amphibia:
- Skin is smooth, glandular and moist.
- Presence of external tympanum.
- Digits are without claws.
- Body with head and trunk but no neck.
- Tadpole larva is aquatic, respiring by gills but in adult, respiration occurs by lungs, skin and buccal cavity.
- Terrestrial or aquatic.
- Examples: Toad (Bufo), Frog (Rana), Limbless amphibia (Ichthyophis), Salamander (Ambystoma).
The class Amphibia includes about 2,500 known species.
(L. repere-to crawl)
Characteristics of Reptilia
- The body is covered with horny epidermal scales.
- The body is divisible usually into the head, neck, trunk and tail.
- Respiration is by lungs only.
- Paired pentadactyl limbs with clawed digits (absent in snakes).
- The embryo is covered by foetal membranes (e. g., amnion and allantois).
- Mostly terrestrial, some aquatic (marine or fresh water).
- Examples: Garden lizard (Calotes), House lizard (Hemidactylus), Crocodile (Crocodilus), Snakes, Turtle, Tortoise etc.
The class Reptilia comprises about 6,300 known species.
Characteristics of Aves
- The body is covered with feathers.
- Forelimbs are modified into wings for flight.
- Warm-blooded (homoiothermic).
- Lungs with air sacs.
- Jaws are with horny beaks, no teeth.
- Hind limb is with four digits adapted for perching, walking or swimming.
- Generally adapted to the aerial mode of life.
- Examples: Pigeon (Columba), Fowl (Gallus). Duck, Ostrich etc.
The class Aves includes about 8.600 known species.
Characteristics of Mammalia
- The body is covered with hairs.
- Presence of mammary glands.
- Presence of external ears (pinna).
- Digits usually ending in claws, nails or hoofs.
- Higher mammals are viviparous (some primitive mammals are oviparous e.g., dull bill platypus).
- The foetus (embryo) is nourished by the mother through a structure, called the placenta.
- Examples of Mammalia: Duck Bill platypus (Ornithorhynchus), Kangaroo (Macropus), Dog(Canis), Tiger (Panthera), Cat (Felis), Horse (Equus), Monkey, Man (Homo).
The class Mammalia comprises about 3,700 known species.