Phylum Chordata – Characteristics and Classification

Phylum Chordata – Characteristics and Classification

All chordates possess these 5 primary characteristics including a notochord, dorsal and hollow nerve cord, endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name is Chordates because of their characteristic feature “notochord”, which plays a significant role in chordate structure and movement. 

Characteristics features of Chordates

The fundamental character of Phylum Chordata is the presence of a notochord, the dorsal hollow nerve cord, and pair of pharyngeal gill slits. The animals are Aquatic, aerial, or terrestrial all free living with no fully parasitic forms.

Important features of the organisms of Phylum Chordata are given below:

• Body symmetry: The members of this phylum possess a bilaterally symmetrical body.
• Germinal layer: Triploblastic: ectoderm, mesoderm, and endoderm.
• Exoskeleton preset and well-developed in most vertebrates.
• Body cavity or coelom: Coelomate
• Complete digestive system and digestive glands.
• Closed Blood vascular system. Heart ventral with dorsal and ventral blood vessels.
  The hepatic portal system is well-developed.
• Level of organization: The organ-system level of organization is present in Chordates.
• The members have a post-anal tail.
• The excretory system comprises proto –or meso- or meta-nephric kidneys.

Comparison of Chordates and Non-chordates

Phylum Chordata is divided into three subphyla:

1. Urochordata or Tunicata:

The notochord is present only in the larval tail. Examples- Ascidia, Salpa, Doliolum.

2. Cephalochordata:

the notochord extends from head to tail region and persists throughout life. Example- Branchiostomata (Amphioxus or lancelet)

***The two subphyla Urochordata and Cephalochordata are exclusively marine and these are referred to as protochordate- (The protochordate closely related to chordates)

3. Vertebrata

Vertebrata (Craniata)– 54,000 species of true chordates

The members possess the notochord during the embryonic period. chordates with the head or skull, brain, and vertebral column. In adults, the notochord is replaced by a cartilaginous or bony vertebral column. There is an important point- all vertebrates are chordates but all chordates are not vertebrates. Some other characters are:

• Heart: Vertebrates possess a ventral muscular heart with two, three, or four chambers.
• Kidney: The kidney functions for excretion and osmoregulation.
• Paired appendages may be in the form of fins or limbs.

Further division of Subphylum Vertebrata

Division of Vertebrata

The Vertebrates are divided into:

1. Agnatha -class→Cyclostomata
2. Gnathostomata →Superclass- Pisces and Tetrapoda
   • Pisces→Class 1. Chondrichthyes 2. Osteichthyes
   • Tetrapoda→Class 1. Amphibia, 2.Reptilia, 3. Aves, 4. Mammals

Division Agnatha: 

Class -Cyclostomata

These are ectoparasites on some fishes. (Ectoparasite: the parasite lives inside or on the skin, but not within the body.)

Features of Class Cyclostomata:

• Their body is elongated with 6-15 pairs of gill slits that function for respiration.
• sucking circular mouth without jaws
• No scales and paired fins on the body.
• Cartilaginous Cranium and vertebral column are present, Brain is seen.
• The heart is two-chambered, Sinus Venosus is present, but CONUS arterescus is absent.
• Closed circulation
• The excretory system includes a pair of Mesonephric Kidneys.
• Cyclostomes are marine but for spawning, they migrate to freshwater.
• They die within a few days after spawning.
• After the metamorphosis, their larvae return to the ocean.
• Examples of cyclostomes: Petromyzon (lamprey and their fossils.), Myxine (Hagfish- Epatratus is a large genus of Hagfish).

1. Petromyzontia– Eary fossils Endeiolepis and Euphenerops, Petromyzon, and modern lamprey.
   • The buccal funnel is suctorial and shows horny teeth.
   • The mouth is present in the buccal funnel
   • Seven pairs of gill slits
   • the brain is well-developed
   • The pineal eye is well-developed
   • The ear has two semicircular canals
2. Myxinoidea–  Myxine- Hagfish- Eel-shaped slime-producing marines.
   • The buccal funnel is absent
   • Eyes are vestigial
   • the dorsal fin is absent or very small
   • Brain is primitive
   • The pineal eye is reduced.
   • The ear consists of only one semicircular canal
   • The hagfishes are marine.

Division- Gnathostomata

Superclass- Pisces- Bear Fins

Class – Chondrichthyes

The Chondrichthyes are marine animals having streamlined bodies. The salient features of Chondrichthyes are:

• The endoskeleton is cartilaginous.
• Mouth ventrally located.
• The notochord is persistent throughout life.
• Gill slits are separate, and the operculum (gill cover) is not found.
• Placoid scales (tiny tough scales) are found.
• Teeth are backwardly directed placoid scales and a powerful jaw.
• The members are predaceous (living by preying on other animals).
• The air bladder is absent so they have to swim constantly to avoid skinning.
• Heart – The two-chambered heart is present.
• In some members, electric organs (Torpedo), and in a few membres poison sting (trygon) is also found.
• These are Poikilothermous i.e. cold-blooded animals, not able to regulate their body temperature.
• Separate sexes
• Pelvic fins bear claspers in males.
• Fertilization is internal and these are mostly viviparous (giving birth to a living young one and developing inside the body of the parent).
• Examples of Chondrichthyes: are Scoliodon (Dogfish), Pristis (sawfish), Carcharodon (great white shark), and Trygon (stingray).

Example of Cartilaginous fishes

(Click here to read about → Scoliodon)

Class-Osteichthyes

Members are both marine and fresh-water fishes consisting of a bony endoskeleton and a streamlined body. Other characteristics  features are:

• The members of this class consisted of mostly terminal mouths.
• Having four pairs of gills covered by an operculum on each side.
• Cycloid or ctenoid scales cover the skin. (Cycloid scales-Large thin and round or oval in shape with grown rings.)
• The air bladder regulates buoyancy.
• Two chambered heart is found i.e. one auricle and one ventricle.
• These animals are cold-blooded.
• Separate sexes are found and usually, the fertilization is external.
• The animals are oviparous and development is direct.
• Some examples are:
  1. Marine– Exocoetus– Flying fish, Hippocampus– sea horse
  2. Fresh water– Labeo– rohu, Catla– Katla, Clarias– Mangur
  3. Aquarium– Betta– Fighting fish, Pterophyllum– Angelfish

Superclass -Tetrapoda

Class-Amphibia (Amphi- dual, bios-life)

The members of this class can live in both aquatic as well as terrestrial habitats. they consist of characteristics features as :

• Mostly these consist of two pairs of limbs.
• The body is divided into the head and trunk. In some members tail is present.
• Their skin is moist without scales.
• Eyes with eyelids. 
• The ear is represented by the tympanum.
• Cloaca:  it is a common chamber in which the alimentary canal, urinary, and reproductive tracts open in the cloaca. The cloaca opens to the exterior of the body.
• Gills, lungs, and skin function for respiration.
• In amphibians, the heart is three chambered i.e. two auricles and one ventricle.
• The blood vascular system is the Hepatic and Renal portal system
• The blood contains nucleated RBC, Haemoglobin is present in the blood.
• These animals are cold-blooded.
• Mesonephric Kidneys are present.
• Well-developed Central nervous system.
• 10 pairs of cranial nerves are present.
• Sexes are separate, fertilization is external.
• They are oviparous and indirect development is found in them.
• The egg is telolecithal, holoblastic cleavage, and unequal.
• Some examples: Bufo– Toad, Rana– Frog, Hyla– Treefrog, Salamandra– Salamander, Ichthyophis– Limbless amphibia.

Class-Reptilia (Latin- Repere or reptum- to creep or crawl)

Class reptilia under Phylum Chordata consist of members that are mostly terrestrial, with the covered body by dry cornified skin, epidermal scales, or scutes. Some other important features are given below:

• Two pairs of limbs (if present).
• External ears not found, tympanum represents the ear.
• Mostly three-chambered hearts, but in Crocodiles it is four-chambered.
• These are poikilotherms.
• Snakes and lizards shed scales as the skin is cast.
• In reptiles, separate sexes are found and fertilization is internal.
• These animals are oviparous and development is direct in them.
• Some examples: Chelone– Turtle, Testudo– Tortoise, Chameleon– Tree lizard, Calotes– Garden lizard, Crocodilus– Crocodile, Alligator– Alligator, Hemidactylus– wall lizard,
  • Poisonous snakes– Naja– Cobra, Bangarus– Karait, Vipera– Viper

Class- Aves

The most important characteristic features of Aves are the presence of feathers and their ability to fly except for some flightless birds like Ostrich. Some other characteristics and features of class Aves are as follows:

• Feather and flying capacity except for a few members.
• The forelimbs of aves are modified into wings.
• Hind limbs consist of scales and are modified for walking, swimming, or clasping on the branches of trees.
• Their skin is dry, absence of a gland except for the one oil gland which is present at the base of the tail.
• fully ossified (bony) endoskeleton, long bones are hollow with air cavities (pneumatic).
• The digestive tract consists of additional chambers called the crop and gizzard.
• Heart of Aves is completely four-chambered.
• The members are warm-blooded i.e. homoiothermic (able to maintain body temperature constant).
• Lungs function as respiratory organs.
• Separate sexes are found.
• Internal fertilization in Aves and these are oviparous animals.
• The development is a direct type.
• Some examples: Corvus– Crow, Columba– Pigeon, Psittacula– Parrot, Struthio– Ostrich, Pavo– Peacock, Aptenodytes– Penguin, Neophron– Vulture

Examples of Aves under Phylum Chordata

Class – Mammalia

The scientific name Mammalia was coined by Carl Linnaeus in 1758.

The members of this class are very advanced and developed. They live in various types of habitats like polar ice caps, deserts, mountains, forests, grasslands, and dark caves. Important features of class mammalian are:

The first mammals appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. Class Mammalia consists of 45,00 species.

• The most unique characteristic of mammals is the presence of mammary glands i.e. milk-producing glands.
• Young ones are nourished by the milk produced by the mammary gland.
• The skin is also a unique feature of mammals as it possesses hair.
• In the members of this class, the external ear called the pinnae is found.
• In jaws different types of teeth are present.
• Some mammals are adapted for living in water or flying.
• These consist of two pairs of limbs by which they are able to walk, run, climb, burrow, swim, or fly.
• The heart is four-chambered. These are homoiothermic.
• Respiratory organs are lungs.
• Separate sexes and internal fertilization. On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species.
• The mammalians are viviparous (birth of a young one developed inside the parent body), and few exceptions are found. The development is direct.

Fig showing the scientific classification of mammals

Sub-Classes of Mammalia:

1. Subclass Prototheria, Order Monotremata:
   • These are egg-laying animals.
   • Teeth are absent in them.
   • The true mammary gland is absent.
   • Found in Australia and New Zealand
2. Subclass Metatheria, Order Marsupialia: 
   • The embryo develops in a pouch
   • prolonged gestation period and after this Parental care
   • Ex. Opossum, Kangaroo, Koala, Tasmanian Devil, Wombat, etc.
3. Subclass Eutheria, Includes 16 orders with 4700 species.
   • Placental Mammals
   • Truly viviparous
   • Placenta for gas and nutrient exchange between the mother and feotus.
   • They possess true mammary glands.

• Some examples of mammalians:
  • Ovipoarous: Ornithorhynchus – Platypus
  • Viviparous: Macropus– Kangaroo, Pteropus– Flying fox, Camelus– Camel, Macaca– Monkey, Rattus– Rat, Canis– Dog, Felis– Cat, Elephas– Elephant, Equus– Horse, Delphinus – Common dolphin, Balaenoptera– Blue whale, Panthera tigris- Tiger, Panthera leo– Lion.

Example of Mammals – Phylum Chordata

You can also read:

• Phylum – Porifera
• Phylum – Coelenterata (Cnidaria)
• Phylum- Ctenophora
• Phylum- Platyhelminthes
•  Aschelminthes,
• Phylum-Annelida
• Phylum-Arthropoda
• Phylum-Mollusca
• Phylum – Echinodermata
• Characters of Archaeopteryx

Reference: NCERT Book and other textbooks.

https://en.wikipedia.org/wiki/Chordate

Thank you 🙂