Phylum Chordata

Animals belong to the largest kingdom Animalia or Animal kingdom. Animals can not make their own food. So, they are dependent on other organisms for nutrition and hence known as heterotrophs. They are multicellular performing different functions. They perform displacement i.e, they can move from one place to another, unlike plants. Animals transfer signals with the help of nerve cells. In sponges nerves cells are absent. The central vacuole is totally absent but a small vacuole maybe occur. Animalia possess different Phylum(Porifera, Coelenterata, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata and Chordata). The term Phylum was coined by Georges Leopold Cuvier in 1869. Let us have a glance at Phylum – Chordata.

Chordata

Diagnostic Characters of Chordates

1. Notochord: It is a solid unjointed, stiff but flexible rod-like structure situated on the mid-dorsal side between the central nervous system and the alimentary canal. The notochord remains throughout like in certain chordates e.g., some protochordate. In vertebrates it is only found in the embryo, however, in adults, it is replaced by a vertebral column (backbone).
2. Dorsal Hollow Nerve Cord: It is always hollow and lies dorsal to the notochord.
3. Paired Pharyngeal Gill Slits: All the chordates have at some stage of life, a series of paired narrow openings and the gill slits on the lateral sides of the pharynx.
4. Tail: It is a postanal part of the body. The tail is absent in many adult chordates. 

Classification of Phylum Chordata

Phylum Chordata is divisible into three sub-phyla- Urochordata, Cephalochordata, and Vertebrata.

Subphylum Urochordata (Tunicata)

• This sub-phylum is also known as Tunicata because the adult body is surrounded by a leathery test or tunic formed of a cellulose-like organic substance entitle tunicin.
• Only the tail of the larva comprises the notochord. It is displaced by a dorsal ganglion in the adult.
• The dorsal tubular nerve cord is present in the larval form and it degenerates in the form of a small ganglion in the adult.
• They are Hermaphrodites.
• The larva (tadpole) undergoes retrogressive metamorphosis, i.e., change from a better-developed larva to a less-developed adult, e.g., Herdmania (sea squirt) Ascidia, Botryllus (colonial urochordate), Molgula, Doliolum, Salpa, Pyrosoma (colonial urochordate), Oikopleura.
• Exclusively marine, solitary, and colonial.
• Examples: Oikopleura, Herdmania, Pyrosoma, Doliolum, Salpa.

Subphylum Cephalochordata

• The notochord extends up to the anterior end of the body hence this subphylum is named.
• The notochord persists throughout life.
• Pharyngeal gill slits are more several and are better developed.
• The atrium is also present.
• The wheel organ of Amphioxus is also called the ciliated organ of the Muller or rotatory organ. Posterior to the wheel organ there is a circular ring-like structure called velum. The velum leads into the pharynx.
• The tail is present throughout life, e.g., Branchiostoma (= Amphioxus) or Lancelet. Subphyla Urochordata and Cephalochordata are collectively called acrania (without cranium brain box) or protochordate (primitive chordates).
• Sexes separate.
• Examples: Amphioxus or Branchiostoma(lancelet).

Subphylum Vertebrata or Craniata

• In the embryonic stage, the notochord is present. It is displaced by the backbone (vertebral column) in adult forms.
• Coupled appendages are certainly not more than two pairs.
• Cephalization (formation of head) can be seen.
• The epidermis consists of various layers of cells. The epidermis may put up with an exoskeleton of scales, feathers, or hair.
• The coelom is well developed.
• Complete digestive tract and is ventral to the central nervous system.
• The endoskeleton is made up of cartilage or of cartilage and bone.
• Ventrally situated hearts and their hepatic portal system are present.
• There is a closed circulatory system containing blood vascular and lymphatic systems. Lymph is like blood but is colorless. Blood is with red and white blood corpuscles. Red blood corpuscles contain hemoglobin.
•  Gills, skin, buccopharyngeal cavity, and lungs are considered respiratory organs.
• Excretion takes place with the help of a pair of kidneys.
• The central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), and autonomic nervous system (sympathetic and parasympathetic nervous systems) make up the nervous system.
• Cranial nerves are 8, 10, or 12 pairs.
• All vertebrates contain endocrine glands.
• They are unisexual, except Hagfish who is bisexual. They lack asexual reproduction.

Classification of Sub-Phylum Vertebrata

Subphylum Vertebrata is divisible into two sections: Agnatha and Gnathostomata.

 Agnatha (Gr., a-without; gnathos -jaws)

• Animals have vertebral columns and cranium. They are the earliest vertebrates known to humans.
• They are without true jaws but have a suctorial mouth.
• Without paired appendages or fins.

Agnatha includes the following single class:

Class Cyclostomata (Gr., kyklos- circle, stoma- mouth)

• The body is long, elongated, and eel-like.
• Skin is soft, slimy, smooth, and scaleless.
• The mouth is round, suctorial, and without jaws. They are ectoparasites and use their mouth to stick to the back of other fishes.
• The single and median nostril is present.
• Respiration is through gills contained in pouches (which are 5 to 15 pairs in hagfishes and 7 pairs of lampreys).
• The cartilaginous endoskeleton is present and the notochord is in the form of a cylindrical rod and continues throughout life.
• The heart is two-chambered.
• Gonad is single and fertilization is external. Development is direct or indirect. 
• Aquatic, marine, and freshwater.
• Free-living and ectoparasites of fishes.
• Examples: Petromyzon (lamprey), Myxine (hagfish), and Bdellostoma (hagfish).

Gnathostomata(Gr., gnathos jaws; stoma- mouth)

• Vertebrates with jaws and paired appendages.
• This subphylum is divisible into the following six classes: class Chondrichthyes and class Osteichthyes, (a combination of these both classes form Pisces.) 

Class 1. Chondrichthyes (Gr. chondros = cartilage+ ichthys a fish; cartilaginous fishes).

Marine fishes with a completely cartilaginous endoskeleton. The mouth is ventral in position. Skin is tough and coated with minute placoid scales. Respiration through gills. 5 or 7 pairs of gills open outside with the help of gill slits. They have fins for locomotion (swimming) and balance. Fins may be paired (pectoral fins, pelvic fins) median (dorsal fin, caudal fin, and anal fin). Tail or caudal fin is heterocercal. The muscular tail is used for movement.

Examples: Scoliodon, Sphyrna, Torpedo, etc.

Class 2. Osteichthyes (Gr. Osteon = bone + ichthys a fish ; Bony fishes).

Marine and fresh-water fish with partly or whole bony endoskeleton include in class Osteichthyes. The body is generally spindle-shaped. Skin is naked or covered with cycloid or ctenoid scales. The mouth is usually terminal (anterior) in position. Four pairs of gills are present and they are coated by the operculum. Gills are filamentous. Fish take in oxygen dissolved in water with the help of gills. They lay eggs and fertilization is external.

Examples: Labeo, Hippocampus, Anabas, etc.

Class 3. Amphibia (Gr., amphi-double; bios-life). 

It includes frogs, toads, newts, and salamanders. These animals live both in fresh water and on land (moist places). Skin is smooth or rough, moist, slimy, glandular, and mostly without scales. Mucus glands are present in the skin. Limbs may be absent in some cases.The heart is three-chambered and has two auricles and one ventricle. Double circulation through the heart. Ectothermic (= cold-blooded animals).

Examples:

• Apoda: Ichthyophis; 
• Urodela: Amphiuma, Salamandra, Ambystoma, Necturus;
• Anura: Rana, Bufo, Hyla, Xenopus laevis.

Class 4. Reptilia (L., repre- to crawl; creeping vertebrates). (Includes lizards, snakes, crocodiles, and tortoises).

Cold-blooded, terrestrial or aquatic vertebrates with a body coated with dry water-proof skin having horny epidermal scales or dermal scute plates. The body ranges in form and is usually divisible into the head, neck, trunk, and tail. The tympanum is small and depressed. Teeth are present in all reptiles, exception (of tortoises and turtles).  Respiration is through the lungs only. No gills are present. Fertilization is internal.

Most reptiles are oviparous and lay their eggs with tough covering and need not to lay their eggs in water. A few reptiles are viviparous for example lizards and snakes. No aquatic larval stage.

Examples: Kachuga, Chelone, Uromastix, Draco, etc.

Class 5. Aves (L., avis-bird).

Warm-blooded, tetrapods vertebrates (birds). Basically, the size extended from the smallest hummingbird to the largest ostrich. Horny scales persist on the feet but the maximum part of the body is covered by feathers. Cutaneous glands are absent. Spindle-or boat-shaped body is divided into a head, neck, trunk, and tail. Fore-limbs are modified into wings for flight. Kiwis have vestigial wings. Hind limbs have four clawed digits and are adapted for walking, perching, or swimming. They show parental care. Fertilization is internal. They are oviparous.

Examples: Gallus, Passer, Corvus, Columba, Psittacula, Pavo, etc.

Class 6. Mammalia (L., mamma-breast).

Mammals are warm-blooded. The body is split into the head, neck, trunk, and tail. Movable eyelids are present. Milk-producing mammary glands are present in females which secrete milk for the feeding of the young ones. Fleshy external ear (pinnae) present. Two pairs of pentadactyl limbs are present. Limbs are variously modified for walking, running, climbing burrowing, swimming, or flying. Respiration is through the lungs only.

Examples: Macropus, Erinaceus, Talpa, Sorex, Pteropus, Bat, Manis, Hystrix, Funambulus, Homo (human being), etc.

FAQs on Chordata

Question 1: What are the 5 main chordate groups?

Answer:

The phylum Chordata is divisible into 5 common classes- fish, amphibians, reptiles, mammals, and aves.

Question 2: What makes chordates different from other animals?

Answer:

A notochord, hollow nerve chord, Gill slits, etc are some distinguishing characteristics that make chordates different from other animals.

Question 3: Which chordate group has the most species?

Answer:

The largest group of Chordata is vertebrates, with more than 62,000 species.

Question 4: What happens to the notochord in vertebrates and protochordate?

Answer:

In vertebrates, the notochord vanishes and produces the spine (vertebral column). In protochordate, the notochord remains during the rest of their life.

Question 5: Give some examples of Phylum Chordata.

Answer:

Pandas, crows, sharks, owls, humans, etc. are some of the examples of Chordata.