Classification of Chordates
The classification of chordates groups all animals that have a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Phylum Chordata has three subphyla, Urochordata, Cephalochordata and Vertebrata, and the vertebrates are divided into seven classes, from Cyclostomata to Mammalia. This page on the classification of chordates gives every feature and example of each class, with comparison tables and figures. NEET regularly asks class features, heart chambers and genus-to-common-name pairs.
- ★ Must learn Chordates: notochord, dorsal hollow nerve cord and paired pharyngeal gill slits; also a post-anal tail and closed circulation.
- Non-chordates: central nervous system ventral, solid and double; gill slits absent; heart dorsal, if present.
- ★ Must learn Subphyla: Urochordata (notochord only in the larval tail), Cephalochordata (head to tail, lifelong), Vertebrata (notochord replaced by a vertebral column).
- All vertebrates are chordates, but all chordates are not vertebrates.
- Agnatha (jawless): Cyclostomata. Gnathostomata (jawed): Pisces (Chondrichthyes, Osteichthyes) and Tetrapoda (Amphibia, Reptilia, Aves, Mammalia).
- ★ Must learn Cyclostomes: ectoparasites on fishes, 6-15 pairs of gill slits, circular sucking mouth without jaws, no scales or paired fins.
- ★ Must learn Chondrichthyes: cartilaginous, placoid scales, no operculum, no air bladder, claspers. Osteichthyes: bony, operculum, air bladder.
- Heart: 2 chambers in fishes; 3 in amphibians and most reptiles; 4 in crocodiles, birds and mammals.
- Cold-blooded: fishes, amphibians, reptiles. Warm-blooded: birds and mammals.
- ★ Must learn Birds: feathers, wings, pneumatic bones, crop and gizzard, air sacs. Mammals: mammary glands, hair, pinnae.
- Mammals are viviparous with few exceptions; Ornithorhynchus (platypus) is oviparous.
1. Phylum Chordata: Fundamental Features
★ Very important Chordates are fundamentally characterised by a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
- Chordates possess a notochord either throughout life or during early embryonic life.
- They are bilaterally symmetrical, triploblastic and coelomate, with the organ-system level of organisation.
- ★ Exam imp They possess a post-anal tail and a closed circulatory system.
Chordate checklist: "Notochord, Nerve cord, Gill slits, Tail", read from the drawing: the rod, the tube above it, the slits in the pharynx and the part behind the anus.
2. Chordates and Non-chordates Compared
| No. | Chordates | Non-chordates |
|---|---|---|
| 1 | Notochord present | Notochord absent |
| 2 | Central nervous system is dorsal, hollow and single | Central nervous system is ventral, solid and double |
| 3 | Pharynx perforated by gill slits | Gill slits are absent |
| 4 | Heart is ventral | Heart is dorsal (if present) |
| 5 | A post-anal part (tail) is present | Post-anal tail is absent |
The two columns are mirror images: chordates have the nerve cord on the back and the heart in front (dorsal nerve cord, ventral heart); non-chordates have the reverse. Hollow and single goes with chordates; solid and double with non-chordates.
3. Subphyla of Chordata
- Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
- ★ Exam imp Urochordata and Cephalochordata are often called protochordates. They are exclusively marine.
Notochord present only in the larval tail.
Examples: Ascidia, Salpa, Doliolum.
Notochord extends from head to tail and persists throughout life.
Example: Branchiostoma (Amphioxus or lancelet).
3.1 Subphylum Vertebrata
- Vertebrates possess a notochord during the embryonic period.
- In the adult, the notochord is replaced by a cartilaginous or bony vertebral column.
★ Very important Thus all vertebrates are chordates, but all chordates are not vertebrates. Protochordates have a notochord but no vertebral column.
- Besides the basic chordate characters, vertebrates have:
- a ventral muscular heart with two, three or four chambers
- kidneys for excretion and osmoregulation
- paired appendages, which may be fins or limbs
Urochordate examples: "All Sea Dwellers" for Ascidia, Salpa, Doliolum; it also recalls that protochordates are exclusively marine. "Uro" means tail, where their notochord lies; "cephalo" means head, where theirs begins.
4. Classification of Subphylum Vertebrata
- Subphylum Vertebrata is divided into two divisions: Agnatha (lacks jaw) and Gnathostomata (bears jaw).
- Agnatha has one class, Cyclostomata.
- Gnathostomata has two super classes: Pisces (bear fins) and Tetrapoda (bear limbs).
- ★ Exam imp Pisces: Chondrichthyes and Osteichthyes. Tetrapoda: Amphibia, Reptilia, Aves and Mammalia.
| Division | Super class | Classes |
|---|---|---|
| Agnatha (lacks jaw) | None | 1. Cyclostomata |
| Gnathostomata (bears jaw) | Pisces (bear fins) | 1. Chondrichthyes 2. Osteichthyes |
| Tetrapoda (bear limbs) | 1. Amphibia 2. Reptilia 3. Aves 4. Mammalia |
"A-gnatha" = without jaw; "gnatho-stomata" = jaw-mouthed. Tetrapoda ("four feet") have two pairs of limbs; Pisces have fins.
5. Class Cyclostomata
- All living members are ectoparasites on some fishes.
- ★ Exam imp They have an elongated body bearing 6-15 pairs of gill slits for respiration.
- Cyclostomes have a sucking and circular mouth without jaws.
- The body is devoid of scales and paired fins.
- The cranium and vertebral column are cartilaginous.
- Circulation is of the closed type.
- Cyclostomes are marine, but they migrate to fresh water for spawning.
- After spawning, they die within a few days.
- Their larvae undergo metamorphosis and return to the ocean.
- Examples: Petromyzon (lamprey) and Myxine (hagfish).
- Agnatha is represented by this class. Cyclostomes are described as the most primitive chordates among the vertebrates, and they are ectoparasites on fishes.
6. Class Chondrichthyes
- They are marine animals with a streamlined body and a cartilaginous endoskeleton.
- The mouth is located ventrally.
- ★ Exam imp The notochord is persistent throughout life.
- Gill slits are separate and without an operculum (gill cover).
- The skin is tough, containing minute placoid scales.
- The teeth are modified placoid scales, which are backwardly directed.
- Their jaws are very powerful. These animals are predaceous.
★ Very important Due to the absence of an air bladder, cartilaginous fishes have to swim constantly to avoid sinking.
- The heart is two-chambered (one auricle and one ventricle).
- Some have electric organs (Torpedo), and some possess a poison sting (Trygon).
- They are cold-blooded (poikilothermous): they lack the capacity to regulate their body temperature.
- Sexes are separate. In males, the pelvic fins bear claspers.
- Fertilisation is internal, and many of them are viviparous.
- Examples: Scoliodon (dog fish), Pristis (saw fish), Carcharodon (great white shark), Trygon (sting ray).
7. Class Osteichthyes
- It includes both marine and fresh water fishes with a bony endoskeleton.
- The body is streamlined. The mouth is mostly terminal.
- ★ Exam imp They have four pairs of gills, covered by an operculum on each side.
- The skin is covered with cycloid/ctenoid scales.
- An air bladder is present, which regulates buoyancy.
- The heart is two-chambered (one auricle and one ventricle). They are cold-blooded.
- Sexes are separate. Fertilisation is usually external.
- They are mostly oviparous, and development is direct.
Examples to Remember
| Habitat | Examples |
|---|---|
| Marine | Exocoetus (flying fish), Hippocampus (sea horse) |
| Freshwater | Labeo (rohu), Catla (katla), Clarias (magur) |
| Aquarium | Betta (fighting fish), Pterophyllum (angel fish) |
| Feature | Chondrichthyes | Osteichthyes |
|---|---|---|
| Endoskeleton | Cartilaginous | Bony |
| Habitat | Marine | Marine and fresh water |
| Mouth | Ventral | Mostly terminal |
| Gill cover | Gill slits separate, no operculum | Four pairs of gills, operculum on each side |
| Scales | Minute placoid scales | Cycloid/ctenoid scales |
| Air bladder | Absent (swim constantly) | Present (regulates buoyancy) |
| Fertilisation | Internal | Usually external |
| Eggs or young | Many viviparous | Mostly oviparous |
To place a fish, check three features: operculum, air bladder and scale type. Present operculum and air bladder with cycloid or ctenoid scales mean a bony fish; placoid scales without them mean a cartilaginous fish.
A fish has placoid scales and claspers on its pelvic fins. Name its class.
Which structure regulates buoyancy in bony fishes?
How many pairs of gills does a bony fish have, and what covers them?
8. Class Amphibia
- As the name indicates (Gr. amphi: dual, bios: life), amphibians can live in aquatic as well as terrestrial habitats.
- Most of them have two pairs of limbs.
- The body is divisible into a head and trunk. A tail may be present in some.
- The amphibian skin is moist (without scales).
- The eyes have eyelids. A tympanum represents the ear.
The alimentary canal, urinary and reproductive tracts open into a common chamber, the cloaca, which opens to the exterior.
- Respiration is by gills, lungs and through the skin.
- ★ Exam imp The heart is three-chambered (two auricles and one ventricle).
- They are cold-blooded animals. Sexes are separate.
- Fertilisation is external. They are oviparous, and development is indirect.
- Examples: Bufo (toad), Rana (frog), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless amphibia).
Examples in order: "Big Rains Help Salamanders Increase" for Bufo, Rana, Hyla, Salamandra, Ichthyophis. The rain recalls their moist skin and life in water.
9. Class Reptilia
- The class name refers to their creeping or crawling mode of locomotion (Latin repere or reptum: to creep or crawl).
- They are mostly terrestrial animals.
- The body is covered by dry and cornified skin, with epidermal scales or scutes.
- They do not have external ear openings. A tympanum represents the ear.
- Limbs, when present, are two pairs. Limbs are absent in snakes.
The heart is usually three-chambered, but four-chambered in crocodiles.
- Reptiles are poikilotherms.
- Snakes and lizards shed their scales as skin cast.
- Sexes are separate. Fertilisation is internal. They are oviparous, and development is direct.
Examples to Remember
| Group | Examples |
|---|---|
| Turtle, tortoise | Chelone (turtle), Testudo (tortoise) |
| Lizards | Chameleon (tree lizard), Calotes (garden lizard), Hemidactylus (wall lizard) |
| Crocodilians | Crocodilus (crocodile), Alligator (alligator) |
| Poisonous snakes | Naja (cobra), Bungarus (krait; also printed as Bangarus), Vipera (viper) |
Poisonous snakes: "Never Bother Vipers" for Naja, Bungarus, Vipera.
Turtle and tortoise: Chelone swims (turtle); Testudo walks on land (tortoise).
10. Class Aves
- ★ Exam imp The characteristic features of Aves (birds) are the presence of feathers and the ability of most of them to fly.
- Flightless birds, such as the ostrich, cannot fly.
- They possess a beak.
- The forelimbs are modified into wings.
- The hind limbs generally have scales and are modified for walking, swimming or clasping tree branches (perching).
- The skin is dry, without glands, except the oil gland at the base of the tail.
The endoskeleton is fully ossified (bony), and the long bones are hollow with air cavities (pneumatic).
- The digestive tract has additional chambers, the crop and the gizzard.
- The heart is completely four-chambered.
- ★ Exam imp They are warm-blooded (homoiothermous): they can maintain a constant body temperature.
- Respiration is by lungs. Air sacs connected to the lungs supplement respiration.
- Sexes are separate. Fertilisation is internal. They are oviparous, and development is direct.
Examples to Remember
| Genus | Common name |
|---|---|
| Corvus | Crow |
| Columba | Pigeon |
| Psittacula | Parrot |
| Struthio | Ostrich |
| Pavo | Peacock |
| Aptenodytes | Penguin |
| Neophron | Vulture |
11. Class Mammalia
- Mammals are found in a variety of habitats: polar ice caps, deserts, mountains, forests, grasslands and dark caves.
- Some of them have adapted to fly or to live in water.
★ Very important The most unique mammalian characteristic is the presence of milk-producing glands, the mammary glands, by which the young ones are nourished.
- They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.
- ★ Exam imp The skin of mammals is unique in possessing hair.
- External ears, or pinnae, are present.
- Different types of teeth are present in the jaw.
- The heart is four-chambered. They are homoiothermous. Respiration is by lungs.
- Sexes are separate, and fertilisation is internal.
- They are viviparous with few exceptions: mammals commonly exhibit viviparity. Development is direct.
Examples to Remember
| Type | Examples |
|---|---|
| Oviparous | 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) |
Mammals in unusual habitats are favourite options: Pteropus flies, Delphinus and Balaenoptera live in water, and Ornithorhynchus lays eggs. All of them still have hair and mammary glands.
12. Vertebrate Classes Compared
| Class | Body covering | Heart | Body temperature | Fertilisation | Eggs or young; development |
|---|---|---|---|---|---|
| Chondrichthyes | Tough skin, placoid scales | 2 chambers | Cold-blooded | Internal | Many viviparous |
| Osteichthyes | Cycloid/ctenoid scales | 2 chambers | Cold-blooded | Usually external | Mostly oviparous; direct |
| Amphibia | Moist skin, no scales | 3 chambers | Cold-blooded | External | Oviparous; indirect |
| Reptilia | Dry, cornified skin; scales or scutes | Usually 3; 4 in crocodiles | Cold-blooded (poikilotherms) | Internal | Oviparous; direct |
| Aves | Feathers; dry skin, oil gland only | 4 chambers | Warm-blooded | Internal | Oviparous; direct |
| Mammalia | Hair; mammary glands | 4 chambers | Warm-blooded | Internal | Viviparous (few exceptions); direct |
Heart chambers rise up the classes: fish 2, frog 3, reptile 3 (crocodile 4), bird 4, mammal 4. Fishes, amphibians and reptiles are cold-blooded; birds and mammals are warm-blooded.
13. Salient Features of All Phyla
- The two tables summarise the distinguishing features of every phylum of the animal kingdom.
(a) Body plan
| Phylum | Level of organisation | Symmetry | Coelom | Segmentation |
|---|---|---|---|---|
| Porifera | Cellular | Various | Absent | Absent |
| Coelenterata (Cnidaria) | Tissue | Radial | Absent | Absent |
| Ctenophora | Tissue | Radial | Absent | Absent |
| Platyhelminthes | Organ (and organ-system) | Bilateral | Absent | Absent |
| Aschelminthes | Organ-system | Bilateral | Pseudocoelomate | Absent |
| Annelida | Organ-system | Bilateral | Coelomate | Present |
| Arthropoda | Organ-system | Bilateral | Coelomate | Present |
| Mollusca | Organ-system | Bilateral | Coelomate | Absent |
| Echinodermata | Organ-system | Radial | Coelomate | Absent |
| Hemichordata | Organ-system | Bilateral | Coelomate | Absent |
| Chordata | Organ-system | Bilateral | Coelomate | Present |
(b) Organ systems and distinctive features
| Phylum | Digestive system | Circulatory system | Respiratory system | Distinctive features |
|---|---|---|---|---|
| Porifera | Absent | Absent | Absent | Body with pores and canals in walls |
| Coelenterata (Cnidaria) | Incomplete | Absent | Absent | Cnidoblasts present |
| Ctenophora | Incomplete | Absent | Absent | Comb plates for locomotion |
| Platyhelminthes | Incomplete | Absent | Absent | Flat body, suckers |
| Aschelminthes | Complete | Absent | Absent | Often worm-shaped, elongated |
| Annelida | Complete | Present | Absent | Body segmentation like rings |
| Arthropoda | Complete | Present | Present | Exoskeleton of cuticle, jointed appendages |
| Mollusca | Complete | Present | Present | External skeleton of shell usually present |
| Echinodermata | Complete | Present | Present | Water vascular system, radial symmetry |
| Hemichordata | Complete | Present | Present | Worm-like with proboscis, collar and trunk |
| Chordata | Complete | Present | Present | Notochord, dorsal hollow nerve cord, gill slits with limbs or fins |
14. Exam Essentials: Pairs, Exceptions and Numbers
- Genus-to-common-name traps: Chelone is the turtle, Testudo the tortoise; Exocoetus is the flying fish, Pteropus the flying fox.
- Fishes with no air bladder (cartilaginous) must swim constantly; bony fishes regulate buoyancy with the air bladder.
- Tympanum represents the ear in both amphibians and reptiles; only mammals have pinnae.
- "All chordates are vertebrates" is false: protochordates have a notochord but no vertebral column.
Exceptions
- Urochordates have a notochord only in the larval tail.
- Cyclostomes are marine but spawn in fresh water, and they have no jaws, scales or paired fins.
- Cartilaginous fishes have no air bladder and no operculum, yet many are viviparous.
- Bony fishes have usually external fertilisation; cartilaginous fishes have internal fertilisation.
- Most amphibians have two pairs of limbs, but Ichthyophis is limbless; snakes also lack limbs.
- The reptile heart is three-chambered except in crocodiles, where it is four-chambered.
- Bird skin has no glands except the oil gland at the base of the tail; the ostrich cannot fly.
- Mammals are viviparous with few exceptions: Ornithorhynchus (platypus) lays eggs.
Numbers to Remember
- 3 fundamental chordate features; 3 subphyla of Chordata.
- 6-15 pairs of gill slits in cyclostomes.
- 4 pairs of gills in bony fishes, with an operculum on each side.
- 2 heart chambers (one auricle, one ventricle) in all fishes.
- 3 heart chambers (two auricles, one ventricle) in amphibians; usually 3 in reptiles.
- 4 heart chambers in crocodiles, birds and mammals.
- 2 pairs of limbs in tetrapods, when limbs are present.
- 7 vertebrate classes: 1 under Agnatha, 2 under Pisces, 4 under Tetrapoda.
Pairs to Match
| Feature | Class or animal |
|---|---|
| 6-15 pairs of gill slits | Cyclostomata |
| Placoid scales | Chondrichthyes |
| Claspers on pelvic fins | Male cartilaginous fishes |
| Electric organ | Torpedo |
| Poison sting | Trygon |
| Operculum | Osteichthyes |
| Cycloid/ctenoid scales | Osteichthyes |
| Air bladder regulating buoyancy | Osteichthyes |
| Cloaca | Amphibia |
| Epidermal scales or scutes | Reptilia |
| Oil gland at the base of the tail | Aves |
| Crop and gizzard | Aves |
| Air sacs supplementing respiration | Aves |
| Mammary glands | Mammalia |
| Pinnae | Mammalia |
Name the two classes grouped under Pisces.
Which reptiles have a four-chambered heart?
What is the common name of Aptenodytes?
15. Quick Revision
- Chordates: notochord, dorsal hollow nerve cord, paired pharyngeal gill slits, post-anal tail, closed circulation.
- Non-chordates: ventral solid double nerve cord, no gill slits, dorsal heart if present.
- Protochordates (Urochordata, Cephalochordata) are exclusively marine.
- Vertebrates: notochord replaced by a vertebral column; ventral heart; kidneys; paired fins or limbs.
- Agnatha: Cyclostomata. Gnathostomata: Pisces and Tetrapoda.
- Cyclostomes: ectoparasites, 6-15 pairs of gill slits, jawless, no scales, die after spawning in fresh water.
- Chondrichthyes: cartilaginous, ventral mouth, placoid scales, no operculum or air bladder, claspers.
- Osteichthyes: bony, terminal mouth, operculum, cycloid/ctenoid scales, air bladder, usually external fertilisation.
- Amphibia: moist skin, eyelids, tympanum, cloaca, three-chambered heart, external fertilisation.
- Reptilia: dry cornified skin, no ear opening, heart 3-chambered (4 in crocodiles), skin cast.
- Aves: feathers, beak, wings, oil gland, pneumatic bones, crop, gizzard, air sacs, warm-blooded.
- Mammalia: mammary glands, hair, pinnae, different teeth, four-chambered heart, mostly viviparous.
- Fishes, amphibians and reptiles are poikilothermous; birds and mammals are homoiothermous.
16. Solved Examples
List I: A. Placoid scales; B. Cloaca; C. Oil gland at the base of the tail; D. Pinnae
List II: I. Aves; II. Mammalia; III. Chondrichthyes; IV. Amphibia
Choose the correct answer.
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-I, C-IV, D-II
(D) A-II, B-IV, C-I, D-III
Answer: (A). Placoid scales: cartilaginous fishes; cloaca: amphibians; oil gland: birds; pinnae: mammals.
A. A notochord is present.
B. The central nervous system is ventral, solid and double.
C. The pharynx is perforated by gill slits.
D. A post-anal tail is present.
E. The heart is dorsal.
Choose the correct answer.
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only
Answer: (A). B and E describe non-chordates: chordates have a dorsal, hollow, single nerve cord and a ventral heart.
A. Class Amphibia; B. Subphylum Vertebrata; C. Division Gnathostomata; D. Super class Tetrapoda
Choose the correct order.
(A) B, C, D, A
(B) B, D, C, A
(C) C, B, D, A
(D) A, D, C, B
Answer: (A). Vertebrata divides into Gnathostomata, which has the super class Tetrapoda, which includes the class Amphibia.
(A) Pristis: saw fish
(B) Clarias: magur
(C) Hyla: tree frog
(D) Testudo: turtle
Answer: (D). Testudo is the tortoise; the turtle is Chelone.
Statement II: In Cephalochordata, the notochord extends from head to tail and persists throughout life.
Choose the correct answer.
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct
Answer: (A). Both describe the protochordates correctly.
(A) Pteropus
(B) Neophron
(C) Ornithorhynchus
(D) Exocoetus
Answer: (A). Pteropus, the flying fox, is a viviparous mammal. Neophron is a bird, Ornithorhynchus an oviparous mammal and Exocoetus a flying fish.
17. Practice Questions
- "All vertebrates are chordates but all chordates are not vertebrates." Justify the statement.Answer: Every vertebrate has a notochord at least in the embryo, so it is a chordate. Urochordates and cephalochordates have a notochord but no vertebral column, so they are chordates but not vertebrates.
- How important is the presence of an air bladder in Pisces?Answer: In bony fishes, the air bladder regulates buoyancy. Cartilaginous fishes lack it, so they must swim constantly to avoid sinking.
- What are the modifications observed in birds that help them fly?Answer: Feathers, forelimbs modified into wings, a fully ossified endoskeleton with hollow pneumatic long bones, air sacs that supplement the lungs, and a completely four-chambered heart with a constant body temperature.
- Could the number of eggs or young ones produced by an oviparous and a viviparous mother be equal? Why?Answer: Usually not. The eggs of oviparous animals develop outside the body, where many are lost, so many eggs are laid. A viviparous mother nourishes fewer young inside her body, and more of them survive.
- Match the following.
Column I: (a) Operculum; (b) Parapodia; (c) Scales; (d) Comb plates; (e) Radula; (f) Hairs; (g) Choanocytes; (h) Gill slits
Column II: (i) Ctenophora; (ii) Mollusca; (iii) Porifera; (iv) Reptilia; (v) Annelida; (vi) Cyclostomata and Chondrichthyes; (vii) Mammalia; (viii) OsteichthyesAnswer: (a)-(viii), (b)-(v), (c)-(iv), (d)-(i), (e)-(ii), (f)-(vii), (g)-(iii), (h)-(vi). - Match List I with List II. List I: A. Exocoetus; B. Pterophyllum; C. Hemidactylus; D. Aptenodytes. List II: I. Penguin; II. Wall lizard; III. Flying fish; IV. Angel fish.
(1) A-III, B-IV, C-II, D-I
(2) A-IV, B-III, C-II, D-I
(3) A-III, B-II, C-IV, D-I
(4) A-I, B-IV, C-II, D-IIIAnswer: (1). Flying fish, angel fish, wall lizard and penguin, in that order. - Read the statements about Chondrichthyes. A. The notochord persists throughout life. B. An operculum covers the gills. C. An air bladder is present. D. Pelvic fins of males bear claspers. E. Teeth are modified placoid scales.
(1) A, D and E only
(2) A, B and C only
(3) B, C and D only
(4) A, C and E onlyAnswer: (1). B and C are features of bony fishes, not of cartilaginous fishes. - Arrange the events in the life of a cyclostome in the correct order. A. Larvae return to the ocean; B. Adults migrate to fresh water; C. Adults spawn and die within a few days; D. Larvae undergo metamorphosis.
(1) B, C, D, A
(2) C, B, D, A
(3) B, D, C, A
(4) A, B, C, DAnswer: (1). Migration, spawning and death, metamorphosis of larvae, return to the ocean. - Which is NOT a feature of Aves?
(1) Pneumatic long bones
(2) Crop and gizzard
(3) Skin with many glands
(4) A completely four-chambered heartAnswer: (3). Bird skin is dry, without glands except the oil gland at the base of the tail. - Statement I: The heart of the crocodile is four-chambered. Statement II: Reptiles are homoiothermous.
(1) Both statements are correct
(2) Both statements are incorrect
(3) Statement I is correct but Statement II is incorrect
(4) Statement I is incorrect but Statement II is correctAnswer: (3). Reptiles are poikilotherms; the heart is four-chambered in crocodiles. - Name an oviparous mammal.Answer: Ornithorhynchus (platypus).
Common Mistakes to Avoid
- Calling every chordate a vertebrate. Urochordates and cephalochordates have a notochord but no vertebral column.
- Giving cyclostomes jaws, scales or paired fins. They have none of these.
- Giving cartilaginous fishes an operculum or an air bladder. Both belong to bony fishes.
- Writing that all reptiles have a three-chambered heart. Crocodiles have a four-chambered heart.
- Describing amphibian skin as scaly. It is moist and without scales.
- Naming Testudo as the turtle. Testudo is the tortoise; Chelone is the turtle.
- Calling all mammals viviparous. Ornithorhynchus (platypus) is oviparous.
- Mixing poikilothermous and homoiothermous. Fishes, amphibians and reptiles are cold-blooded; birds and mammals are warm-blooded.
Frequently Asked Questions
What are the three fundamental characters of chordates?
Chordates are characterised by a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. They also have a post-anal tail and a closed circulatory system. They are bilaterally symmetrical, triploblastic and coelomate, with the organ-system level of organisation, and their heart is ventral.
What are protochordates?
Protochordates are the subphyla Urochordata (Tunicata) and Cephalochordata, which are exclusively marine. In urochordates such as Ascidia, Salpa and Doliolum, the notochord is present only in the larval tail. In cephalochordates such as Branchiostoma (Amphioxus or lancelet), it runs from head to tail and persists throughout life.
Why are all vertebrates chordates but not all chordates vertebrates?
Vertebrates have a notochord during the embryonic period, which is replaced by a cartilaginous or bony vertebral column in the adult, so every vertebrate is a chordate. Protochordates have a notochord but never form a vertebral column, so they are chordates without being vertebrates.
What is the difference between Chondrichthyes and Osteichthyes?
Chondrichthyes are marine fishes with a cartilaginous endoskeleton, a ventral mouth, placoid scales, gill slits without an operculum, no air bladder and internal fertilisation. Osteichthyes have a bony endoskeleton, a mostly terminal mouth, four pairs of gills covered by an operculum, cycloid or ctenoid scales, an air bladder and usually external fertilisation.
Why do cartilaginous fishes have to swim constantly?
Cartilaginous fishes such as Scoliodon and Pristis lack an air bladder. Without it they cannot regulate their buoyancy, so they must swim constantly to avoid sinking. Bony fishes have an air bladder, which regulates buoyancy and lets them stay at one depth without swimming.
Which features of birds help them fly?
Birds have feathers and forelimbs modified into wings. Their endoskeleton is fully ossified, with hollow, air-filled pneumatic long bones. Air sacs connected to the lungs supplement respiration. A completely four-chambered heart and a constant body temperature support their active life. Flightless birds such as the ostrich cannot fly.
Which mammal lays eggs?
Ornithorhynchus, the platypus, is an oviparous mammal. Mammals are viviparous with few exceptions, and the platypus is the standard example. Like all mammals, the platypus has mammary glands and hair. Viviparous examples include Macropus (kangaroo), Pteropus (flying fox), Delphinus (common dolphin) and Balaenoptera (blue whale).
Which chordate points are most asked in NEET?
NEET most often asks heart chambers, cold- and warm-blooded classes, the features that separate cartilaginous from bony fishes, protochordate notochord position, and genus-to-common-name pairs such as Chelone and Testudo. These usually come as match-the-list questions and statement sets with one or two exceptions hidden among true lines.
Previous year questions on Classification of Chordates
15 questions from past papers, each with a step-by-step solution.
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