Phylum Chordata | Zoology Optional Notes for UPSC PDF Download

Chordata

Diagnostic Characters of Chordates

• Notochord:. solid, unjointed, stiff yet flexible rod-like structure located on the mid-dorsal side, between the central nervous system and the alimentary canal. In some chordates, like certain protochordates, the notochord persists throughout life. In vertebrates, it is present only in the embryo and is later replaced by the vertebral column in adults.
• Dorsal Hollow Nerve Cord: This nerve cord is always hollow and situated dorsal to the notochord.
• Paired Pharyngeal Gill Slits: All chordates possess a series of paired narrow openings, known as gill slits, on the lateral sides of the pharynx at some stage of their life.
• Tail:. post-anal extension of the body. While the tail is present in many chordates during some stage of life, it is absent in many adult forms.

Classification of Phylum Chordata

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

Subphylum Urochordata (Tunicata):

• This sub-phylum is known as Tunicata because the adult body is encased in a leathery test or tunic made of a cellulose-like substance called tunicin.
• In the larval stage, the notochord is present only in the tail. In adults, it is replaced by a dorsal ganglion.
• The dorsal tubular nerve cord is present in the larval form but degenerates into a small ganglion in the adult.
• Urochordates are hermaphrodites, possessing both male and female reproductive organs.
• The larva, known as a tadpole, undergoes retrogressive metamorphosis, transforming from a more developed larva into a less developed adult. Examples include Herdmania (sea squirt), Ascidia, Botryllus (colonial urochordate), Molgula, Doliolum, Salpa, Pyrosoma (colonial urochordate), and Oikopleura.
• Urochordates are exclusively marine and can be solitary or colonial.
• Examples include Oikopleura, Herdmania, Pyrosoma, Doliolum, and Salpa.

Subphylum Cephalochordata:

• The notochord extends to the anterior end of the body, which is why this subphylum is named Cephalochordata.
• The notochord persists throughout the organism's life.
• Pharyngeal gill slits are more numerous and better developed in this subphylum.
• An atrium is also present in these organisms.
• The wheel organ of Amphioxus, also known as the ciliated organ of Muller or rotatory organ, is located posterior to the wheel organ. A circular ring-like structure called the velum leads from the wheel organ to the pharynx.
• The tail is present throughout life in members of this subphylum. Examples include Branchiostoma, also known as Amphioxus or lancelet.
• Subphyla Urochordata and Cephalochordata are collectively referred to as acrania (lacking a cranium or brain box) or protochordates (primitive chordates).

Subphylum Vertebrata or Craniata:

• During embryonic development, the notochord is present but is later replaced by the vertebral column in adult forms. 
• Vertebrates have no more than two pairs of coupled appendages. 
• Cephalization, or the formation of a head, is a characteristic feature. 
• The epidermis consists of multiple layers of cells and may be covered by an exoskeleton made of scales, feathers, or hair. 
• he coelom is well-developed in vertebrates. 
• The digestive tract is complete and located ventral to the central nervous system. 
• The endoskeleton is composed of cartilage or a combination of cartilage and bone. 
• Hearts are located ventrally, and a hepatic portal system is present. 
• Vertebrates have a closed circulatory system that includes blood vascular and lymphatic systems. Lymph is similar to blood but is colorless, while blood contains red and white blood corpuscles. Red blood corpuscles contain hemoglobin. 
• Respiratory organs include gills, skin, buccopharyngeal cavity, and lungs. 
• Excretion is facilitated by a pair of kidneys. 
• The nervous system comprises the central nervous system (brain and spinal cord), peripheral nervous system (cranial and spinal nerves), and autonomic nervous system (sympathetic and parasympathetic nervous systems). 
• Vertebrates have eight, ten, or twelve pairs of cranial nerves. 
• All vertebrates possess endocrine glands. 
• Most vertebrates are unisexual, except for hagfish, which are bisexual. Asexual reproduction is not observed in vertebrates.

Classification of Sub-Phylum Vertebrata

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

• Animals with vertebral columns and cranium.
• Earliest vertebrates known to humans.
• Lacking true jaws, they have a suctorial mouth.
• No paired appendages or fins.
• Includes the class Cyclostomata.

Class Cyclostomata

• Body Structure: Long, elongated, and eel-like.
• Skin: Soft, slimy, smooth, and scaleless.
• Mouth: Round, suctorial, and without jaws; used for sticking to other fishes.
• Nostrils: Single and median nostril present.
• Respiration: Through gills contained in 5 to 15 pairs of pouches (in hagfishes) or 7 pairs (in lampreys).
• Endoskeleton: Cartilaginous, with a notochord in the form of a cylindrical rod, continuing throughout life.
• Heart: Two-chambered.
• Gonad: Single, with external fertilization. Development is direct or indirect.
• Habitat: Aquatic, marine, and freshwater.
• Lifestyle: Free-living and ectoparasites of fishes.
• Examples: Petromyzon (lamprey), Myxine (hagfish), Bdellostoma (hagfish).

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

• Vertebrates with jaws and paired appendages.
• Divisible into six classes: Class Chondrichthyes and Class Osteichthyes (together forming Pisces ).

Class 1. Chondrichthyes:

• Definition: Marine fishes with a completely cartilaginous endoskeleton.
• Mouth Position: Ventral (underneath) the body.
• Skin: Tough and coated with minute placoid scales.
• Respiration: Through gills, with 5 or 7 pairs of gills opening outside via gill slits.
• Fins: Used for locomotion and balance, can be paired (pectoral and pelvic fins) or median (dorsal, caudal, and anal fins).
• Tail: Heterocercal tail (caudal fin) used for movement.

Examples: Scoliodon, Sphyrna, Torpedo, etc.

Class 2. Osteichthyes:

• Definition: Marine and freshwater fish with a partly or wholly bony endoskeleton.
• Body Shape: Generally spindle-shaped.
• Skin: Naked or covered with cycloid or ctenoid scales.
• Mouth Position: Usually terminal (at the front) of the body.
• Gills: Four pairs of gills coated by the operculum, with filamentous gills.
• Respiration: Taking in oxygen dissolved in water through gills.
• Reproduction: Laying eggs with external fertilization.

Examples: Labeo, Hippocampus, Anabas, etc. 

Class 3. Amphibia:

• Definition: Includes frogs, toads, newts, and salamanders.
• Habitat: Live both in freshwater and on land (moist places).
• Skin: Smooth or rough, moist, slimy, glandular, and mostly without scales; mucus glands present in the skin.
• Limbs: May be absent in some species.
• Heart: Three-chambered with two auricles and one ventricle.
• Circulation: Double circulation through the heart.
• Temperature Regulation: Ectothermic (cold-blooded animals).

Examples:

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

Class 4. Reptilia:

• Definition: Includes lizards, snakes, crocodiles, and tortoises.
• Temperature Regulation: Cold-blooded.
• Habitat: Terrestrial or aquatic.
• Skin: Coated with dry, waterproof skin having horny epidermal scales or dermal scute plates.
• Body Structure: Divisible into head, neck, trunk, and tail.
• Tympanum: Small and depressed.
• Teeth: Present in all reptiles, except tortoises and turtles.
• Respiration: Through lungs only; gills are absent.
• Fertilization: Internal.
• Reproduction: Most reptiles are oviparous (lay eggs with tough covering, not requiring water). A few, like lizards and snakes, are viviparous. No aquatic larval stage.

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

Class 5. Aves:

• Definition: Warm-blooded tetrapod vertebrates (birds).
• Body Size: Ranges from the smallest hummingbird to the largest ostrich.
• Skin: Horny scales on feet; most of the body covered by feathers.
• Glands: Cutaneous glands are absent.
• Body Structure: Spindle- or boat-shaped body divided into head, neck, trunk, and tail.
• Fore-limbs: Modified into wings for flight (e.g., Kiwis have vestigial wings).
• Hind Limbs: Four clawed digits adapted for walking, perching, or swimming.
• Parental Care: Show parental care.
• Fertilization: Internal.
• Reproduction: Oviparous (laying eggs).

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

Class 6. Mammalia:

• Definition: Warm-blooded animals.
• Body Structure: Divided into head, neck, trunk, and tail.
• Eyes: Movable eyelids present.
• Glands: Milk-producing mammary glands in females, which secrete milk for feeding young.
• Ears: Fleshy external ears (pinnae) present.
• Limbs: Two pairs of pentadactyl limbs, variously modified for walking, running, climbing, burrowing, swimming, or flying.
• Respiration: Through lungs only.

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