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Introduction to Phylum Chordata
Phylum Chordata (Gr., chorda – string)
Chordates are characterized by the following three features:
- a dorsal, hollow, tubular nerve cord.
- a pliable rod called a notochord that occurs ventral to the nerve cord and is replaced by a bone or cartilage to form a vertebral column in vertebrates.
- paired gill slits in the pharynx. These three characters are always present at some stage in development or throughout a lifetime.
The notochord (Gr. noton = back + L. chorda = cord) is a long rod-like support structure that runs along the back of an animal and separates the nervous tissue from the gut. It provides a space for muscles to attach for ease of movement.
Notochord is a primitive type of endoskeleton that is characteristic among all chordates, at some stage of the life cycle. The notochord provides longitudinal stiffening of the main body axis, a base for trunk muscles, and an axis around which the vertebral column develops. Histologically, cells of the notochord proper are thick-walled, pressed together closely, and filled with semifluid. Stiffness is caused mainly by the turgidity of fluid-filled cells and surrounding connective sheaths.
2. Body bilaterally symmetrically, metarnerically segmented, triploblastic, and coelomate.
3. A post-anal tail is present at some stage of life.
4. Segmental muscles in an unsegmented trunk.
5. Ventral heart, with dorsal and ventral blood vessels and closed blood vascular system.
6. Complete digestive system.
7. A cartilaginous or bony endoskeleton is present in the majority of members (Vertebrates).
Chordata is the most advanced group of animals. Phylum Chordata is divided into two groups, viz., Protochordata (Acrania) and Vertebrata (Craniata).’Protochordates may not have a proper notochord present during all stages of their lives. Protochordata includes two subphyla, Urochordata and Cephalochordata.
Group Vertebrata is divided into two subphyla, viz., Agnatha and Gnathostomata. Agnatha includes a single class Cyclostomata, while Gnathostomata is divided into six classes: Chondrichthyes,, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. Chondrichthyes and Osteichthyes, both are commonly known as (fishes).
Protochordate – Characteristics, Classifications, and Examples
Subphylum 1. Urochordata (Gr., oura- tail), or Tunicata
1. They are bag-like, sessile, soft-bodied, non-metameric animals.
2. Adult body is covered with a tunic or test containing cellulose.
3. Notochord is present only in the tail of a free-living tadpole-like larva. A sessile adult has no notochord.
4. Dorsal tubular nerve cord is present in the larval form and it degenerates in the form of a small ganglion in the adult.
5. The only chordate character present in the adult animal is the presence of numerous gill slits in the pharynx.
7. Exclusively marine, solitary and colonial.
Examples: Oikopleura, Herdmania, Pyrosoma, Doliolum, Salpa.
Subphylum 2. Cephalochordata (Gr., kephale- head)
1. Fish-like, metamerically segmented, headless coelomate animals which retain notochord, dorsal tubular nerve cord, and pharyngeal gill slits throughout life.
2. Notochord and nerve cord (without a distinct brain) extend throughout the entire length of the body. Possesses post-anal tail.
3. Exoskeleton, head, jaws, and paired fins are absent.
4. Pharynx is large and perforated by numerous gill slits.
5. Sexes separate.
6. Marine, burrow in the sand of shallow seawater.
Examples: Amphioxus or Branchiostoma (lancelet).
Classification of Vertebrata (Phylum Chordata)
Subphylum 1. Agnatha (Gr., a- without; gnathos – jaws)
- Animals have vertebral columns and a cranium. They are the most primitive vertebrates known to humans.
- Without true jaws; but possess a suctorial mouth.
- Without paired appendages or fins.
Agnatha includes the following single class:
Class Cyclostomata (Gr., kyklos – circle, stoma – mouth)
1. Body is long, elongated, and eel-like.
2. Skin is soft, slimy, smooth, and scaleless.
3. Mouth is round, suctorial, and without jaws. They are ectoparasites and use their mouth to stick to the back of other fishes.
4. Single and median nostril is present.
5. Respiration is through gills contained in pouches (which are 5 to 15 pairs in hagfishes and 7 pairs in
6. Endoskeleton is cartilaginous and the notochord is in the form of a cylindrical rod and persists throughout life.
7. Fleart is two-chambered.
8. Gonad is single and fertilization is external. Development is direct or indirect. Petromyzon has an ammocoete larva in its life cycle.
9. Aquatic, marine, and freshwater.
10. Free-living and ectoparasites of fishes.
Examples: Petromyzon (lamprey), Myxine (hagfish), and Bdellostoma (hagfish).
Subphylum 2. Gnathostomata (Gr., gnathos- jaws; stoma- mouth)
1. Vertebrates with jaws and paired appendages.
2. This subphylum is divided into the following six classes: class Chondrichthyes and class Osteichthyes, (both are combined together to form a superclass, called Pisces.)
Class 1. Chondrichthyes (Gr. chondros = cartilage + ichthy = a fish; cartilaginous fishes)
1. Marine fish with a completely cartilaginous endoskeleton. They are generally large in size (up to 10 – 20 metres long).
2. Streamlined body is either laterally compressed and spindle-shaped or dorsoventrally flattened and disc-shaped.
3. Mouth is ventral in position.
4. Skin is tough and covered with minute placoid scales.
5. Respiration through gills. Five or seven pairs of gills open outside by means of gill slits. The gill slits are not covered by gill cover or operculum. Without a swim bladder or lungs.
6. They have fins for locomotion (swimming), and balance. Fins may be paired (pectoral fins, pelvic fins) or median (dorsal fin, caudal fin, and anal fin).
7. Tail or caudal fin is heterocercal. The muscular tail is used for movement.
8. Heart is two-chambered.
9. Digestive system has a J-shaped stomach (stomach absent in chimaeras) and the intestine has spiral valves.
10. Sense of smell, vibration reception ( lateral line system), and electro reception are well developed.
11. Alimentary canal and urinogenital ducts open into a common aperture, called cloaca.
12. Excrete urea (ureotelic animals).
Examples: Scoliodon (dogfish, Indian shark), Sphyrna (hammer-headed shark), Torpedo (electric ray), Trygon (sting ray), Pristis (saw-fish), Chimaera (ratfish; Hickman et al. 1996).
Class 2. Osteichthyes (Gr. Osteon bone + ichthys = a fish; Bony fishes)
1. Marine and fresh-water fishes with partly or whole bony endoskeletons.
2. Size varies from 10 mm to 4 metres.
3. Body is generally spindle-shaped.
4. Skin is either naked or covered with cycloid or ctenoid scales.
5. Mouth is usually terminal (anterior) in position.
6. Four pairs of gills are present and they are covered by operculum. Gills are filamentous. Fish obtain oxygen dissolved in water by using gills.
7. Tail or caudal fin is homocercal.
8. Cloaca is absent. Anus and urinogenital apertures are distinct.
9. Excrete ammonia (ammonotelic animals).
10. Heart is two-chambered containing one auricle and one ventricle.
11. They are cold-blooded (ectothermic) animals.
12. They lay eggs and fertilization is external.
Examples: Labeo (rohu, carp), Synchiropus splendidus (mandarin fish), Caulophyryne jordani (angler fish), Pterois volitans (lionfish), Hippocampus (sea horse), Exocoetus (flying fish), Anabas (climbing perch), Protopterus (lungfish).
Differences between Bony Fishes and Cartilaginous Fishes
|Bony Fishes||Cartilaginous Fishes|
|1. They have a bony endoskeleton.||1. They have a cartilaginous endoskeleton.|
|2. They contain four pairs of gill slits.||2. They contain 5-7 pairs of gill slits.|
|3. Their gill slits are covered with an operculum.||3. An operculum is absent in them.|
|4. Their mouth is terminal in position.||4. Their mouth is ventral in position.|
|5. Their caudal fin is homocercal.||5. Their caudal fin is heterocercal.|
|6. Air bladder is usually present in them.||6. Swim bladder is absent in them.|
|7. Examples: Carp, sea horse, etc.||7. Examples: Indian shark, electric ray, etc.|
Class 3. Amphibia (Gr., amphi – double; bios – life).
It includes frogs, toads, newts, and salamanders.
1. In evolutionary terms, amphibians form the first group among the chordates to live outside water and comprise of first four-legged (tetrapod) land vertebrates. They live on land but lay their eggs in water. Amphibians are vertebrates that lead two lives.
2. These animals live partly in fresh water and partly on land (moist places).
3. Skin is smooth or rough, moist, slimy, glandular, and mostly without scales. The skin has mucus glands.
4. Body has a distinct head and trunk and has no neck. The mouth is usually large.
5. Two pairs of pentadactyl (five-digit) limbs are present. Digits or toes are without claws. Limbs may be absent in some cases.
6. Three-chambered heart has two auricles and one ventricle. Double circulation through the heart.
7. Respiration either by gills, lungs, skin, or the mouth lining.
8. Excrete either ammonia (tadpole larva) or urea.
9. Oviparous, lay yolk-laden eggs with gelatinous covering usually in water. Fertilization is external in frogs and toads, but internal in salamanders and apoda. Metamorphosis is present.
10. Ectothermal (= cold-blooded animals).
Examples: 1. Apoda: Ichthyophis; 2. Urodela: Amphiuma, Salamandra, Ambystoma, Necturns; 3. Anura: Ram (bullfrog), Bufo (toad), Hyla (tree frog), Xenopus laevis (African clawed frog).
Class 4. Reptilia (L., repre – to crawl; creeping vertebrates)
Includes lizards, snakes, crocodiles, and tortoises.
1. Ectothermic or cold-blooded, terrestrial, or aquatic vertebrates with a body covered with dry water-proof skin having horny epidermal scales or dermal scute plates.
2. Body varies in form and is usually divided into a head, neck, trunk, and tail.
3. Limbs are tetrapodous pentadactyl (five-toed) type, with clawed digits (limbs are absent in snakes and some lizards).
4. Tympanum is small and depressed.
5. Teeth are present in all reptiles except in tortoises and turtles.
6. Respiration is through the lungs only. No gills are present.
7. Heart is three-chambered and is divided into two auricles and an incompletely divided ventricle. Only crocodiles have four-chambered hearts.
8. Terrestrial reptiles such as snakes and lizards excrete uric acid (uricotelic animals).
9. Fertilization is internal.
10. Most reptiles are oviparous and lay their eggs with tough covering and do not need to lay their eggs in water. A few reptiles are viviparous (e.g., lizards and snakes). No aquatic larval stage.
Examples: Kachuga (roofed terrapin), Testudo (land tortoise), Chelone (green turtle), and Uromastix (sand- lizard). Hemidactylus (house wall-lizard), Calotes (garden-lizard), Draco (flying-lizard), Chamaeleon, Varanus (Indian monitor), Python (ajgar), Naja (cobra), Crocodilus (mugger), Gavialis (gharial).
Differences between Amphibia and Reptilia
|1. Their skin is glandular, smooth, and moist.||1. Their skin is non-glandular, dry, and keratinized.|
|2. Scales are not present in them.||2. Horny scales are present over their body surface.|
|3. Their digits do not possess claws.||3. Their digits end in claws.|
|4. They have a three-chambered heart.||4. Their heart is incompletely four-chambered.|
|5. Fertilization is external.||5. Fertilization is internal.|
|6. Extra-embryonic membranes (e.g., amnion, allantois and chorion) are absent.||6. Extra-embryonic membranes are present in them.|
|7. Their eggs have a soft covering.||7. Their eggs have a hard covering or shell.|
|8. Examples: Frog, toad.||8. Example: Lizard, snake, tortoise, dinosaurs (extinct).|
Class 5. Aves (L., avis – bird)
1. Warm-blooded, tetrapodous vertebrates (birds) with various flight adaptations.
2. Size ranges from smallest hummingbird to largest ostrich.
3. Horny scales persist on the feet but feathers cover most of the body. Cutaneous glands are absent.
4. the Spindle-or boat-shaped body is divisible into head, neck, trunk, and tail.
5. Fore-limbs modified into wings for flight. Kiwis have vestigial wings.
6. Hind limbs bear four clawed digits and are adapted for walking, perching, or swimming.
7. Narrow jaws form a horny beak, which is modified for different purposes. Teeth are absent.
8. Bones of the endoskeleton are light and spongy due to the presence of air cavities.
9. They breathe through the lungs.
10. Four-chambered heart with two auricles and two ventricles.
11. Cloaca is present.
12. Excrete semisolid urine having uric acid (uricotelic animals). No bladder.
13. Birds have highly developed voices, hearing, and vision. They also have a specialized sense for nest building and care for the young (parental care).
14. Fertilization is internal. They are oviparous and lay large, yolk-laden eggs having a hard shells.
Examples: Gallus (chicken), Passer (house sparrow), Corvus (crow), Columba (pigeon), Psittacula (parrot), Pavo (peafowl, peacock), Eudynamys (koel), Bubo (owl), Milvus (kite), Gypus (vulture), Struthio (ostrich), Coracias (blue jay or Neelkanth), Apteryx (kiwi), Aptenodytes (emperor penguin).
Class 6. Mammalia (L., mamma – breast)
1. Mammals are warm-blooded (endothermic and homeothermic) and the most evolved animals of the Animal Kingdom.
2. Hairs and subcutaneous fat form an insulating layer. Cutaneous glands such as sweat glands, scent glands, and sebaceous (oil) glands are present.
3. Body is divisible into a head, neck, trunk, and tail. Movable eyelids are present.
4. Females have milk-producing mammary glands which secrete milk for the nourishment of the young.
5. Fleshy external ear (pinnae) present. Middle ear with three ossicles (malleus, incus, and stapes).
6. Teeth are thecodont (embedded in sockets in the jaws) and are heterodont (differentiated into incisors, canines, premolars, and molars).
7. Two pairs of pentadactyl limbs are present. Digits in the fore-limbs and hind limbs are never more than five and end in claws, nails, or hoofs. Limbs are variously adapted for walking, running, climbing, burrowing, swimming, or flying.
8. A muscular diaphragm separates thoracic and abdominal cavities.
9. Respiration is through the lungs only.
Differences between Aves and Mammalia
|1. Forelimbs are modified into wings.||1. Wings are absent except in bats.|
|2. The body is covered with feathers and scales.||2. Feathers and scales are absent. Hair is present.|
|3. Skin is dry only a single preen gland is present.||3. Skin bears a number of sweat and oil glands.|
|4. Mammary glands are absent.||4. Females have mammary glands for feeding the young.|
|5. A diaphragm is absent.||5. A muscular partition called a diaphragm is present between the thorax and abdomen.|
|6. A toothless beak is present.||6. Jaws do not form beaks. Teeth are present.|
|7. Bones of birds are hollow or pneumatic.||7. Bones of mammals do not possess air cavities.|
|8. Larynx of birds is non-functional. Instead, the syrinx is present.||8. Larynx of mammals is functional. Syrinx is absent.|
|9. Lungs contain external air sacs.||9. External air sacs do not occur in the lungs.|
|10. Eggs possess a great amount of yolk. They are called acrolectal eggs.||10. Eggs have no or a tiny amount of yolk, such eggs art called isolecithal eggs.|
|11. Birds are oviparous.||11. Mammals are viviparous with the exception of prototherians.|
10. Heart is four-chambered. Nonnucleated biconcave red blood corpuscles are present.
11. Excrete urea (ureotelic animals).
12. Penis is always present. Fertilization is internal.
13. Most mammals familiar to us, produce live young, i.e., they are viviparous. In them, the young develop in the uterus of a mother for some time, receiving nourishment and oxygen through the placenta and disposing of its wastes by way of maternal circulation. However, a few of them (such as the platypus and the echidna) lay eggs, i.e., they are oviparous. Still, some others, such as kangaroos, give birth to very poorly developed young.
14. Parental care is highly developed.
15. Mammals live in all kinds of habitats. They are primarily terrestrial animals.
Examples: Macropus (kangaroo), Erinaceus (hedgehog), Talpa (mole), Sorex (shrew), Pteropus (flying fox), Bat, Manis (scaly ant-eater), Hystrix (porcupine), Funambulus (squirrel), Rattus (rat), Oryctolagus (rabbit), Felis (cat), Panthera (lion, tiger, leopard), Canis (dog), Camelus (camel), Rhinoceros, Elephas (elephant), Ursus (bear), Balaena (whale), Macaca (monkey), Homo (human being).