History of animal classification, non-chordates and chordates
A library has thousands of books, yet you find one easily because they are sorted by subject and shelf. Scientists do the same for about 7 million kinds of animals, grouping them by what is alike and what is different.
Animal classification (प्राण्यांचे वर्गीकरण) is forming groups and sub-groups of animals on the basis of similarities and differences.
Rule 1: Why we classify — Classification makes study easy, helps identification, and shows relationships, habitats, adaptations and evolution.
- Easy : Spotting a frog in a pond is easier once you know it belongs to the amphibians.
- Medium : A bat flies like a bird but has hair and feeds milk to its young, so it sits with the mammals.
- Exam-type : A dolphin lives in water like a fish, but it breathes with lungs and has mammary glands, so it is a mammal.
Rule 2: History of classification — Aristotle first classified animals by body size, habits and habitats: the artificial method (कृत्रिम पद्धत), also followed by Theophrastus, Pliny, John Ray and Linnaeus. Later the natural system used body organization, cell types, chromosomes and biochemical properties; then systems based on evolution (Dobzhansky, Meyer) and Carl Woese.
- Easy : Aristotle's way: grouping by where animals live, such as water or land.
- Medium : Under the artificial method a bat and a bird fall together because both fly; the natural system separates them by body organization.
- Exam-type : Natural classification of a whale uses its lungs, hair and milk glands, not the fact that it swims.
Rule 3: Chordates and non-chordates — On the basis of the notochord (पृष्ठरज्जू), animals are non-chordates (no notochord, no pharyngeal gill-slits, solid paired ventral nerve cord, dorsal heart if present; ten phyla) or chordates (notochord, gill-slits or lungs, hollow dorsal nerve cord, ventral heart). Phylum Chordata has three subphyla: Urochordata, Cephalochordata and Vertebrata.
In the figure : In the figure the notochord separates chordates from non-chordates.
- Easy : An earthworm has no notochord, so it is a non-chordate.
- Medium : A fish has a notochord at some stage, a dorsal hollow nerve cord and gill slits, so it is a chordate.
- Exam-type : Check a new animal: no notochord, solid nerve cord on the belly side means non-chordate; notochord with hollow dorsal nerve cord means chordate.
Notochord decides chordate or non-chordate. Aristotle started classification.
Thinking animals that look alike or live in the same place are always related. Group by body features.
Memory clue: "Rod on the back means chordate".
Spot the difference
| Feature | Non-chordates | Chordates |
|---|---|---|
| Notochord | Absent | Present |
| Nerve cord | Solid, ventral | Hollow, dorsal |
| Heart | Dorsal if present | Ventral |
Practice questions
- 1About how many animal species are estimated on earth?
- 2Who first classified animals?
- 3What is the basis of the artificial method?
- 4Name the three subphyla of Chordata.
- 5Name two benefits of classification.
- 6An animal has a hollow dorsal nerve cord and a ventral heart. Chordate or non-chordate?
- 7Why is a bat not grouped with birds even though both fly?
- 8How does the natural system differ from the artificial method?
Show answers (try them yourself first)
- 1About 7 million.
- 2Aristotle.
- 3Body size, habits and habitats.
- 4Urochordata, Cephalochordata and Vertebrata.
- 5It makes study easy and shows relationships between animals.
- 6Chordate.
- 7Natural classification uses body features; a bat has hair and mammary glands.
- 8It uses body organization, cell types, chromosomes and biochemistry, not just habits or habitat.
Grades of organization and body symmetry
A hut has one room where everything happens; a bungalow has separate rooms for sleeping, cooking and studying. Animal bodies are similar: simple ones do everything in one cell, advanced ones have organs and systems.
Grades of organization (संघटन पातळी) show how complex the body is; symmetry (सममिती) shows how the body parts are arranged.
Rule 1: Grades of organization — In Whittaker's five-kingdom system all multicellular animals are in Kingdom Animalia. The grades are: protoplasmic (one cell does all work: Amoeba, Paramecium); cellular (cells, no tissues: Porifera); cell-tissue (Cnidaria); tissue-organ (flatworms such as Planaria); organ-system (crab, frog, human and all the rest).
In the figure : In the figure body organization becomes more complex.
- Easy : Amoeba is a single cell that eats, breathes and moves by itself: protoplasmic grade.
- Medium : A sponge has many cells doing different jobs but no real tissues: cellular grade.
- Exam-type : A frog has organs grouped into systems such as the digestive and respiratory systems: organ-system grade.
Rule 2: Body symmetry — Asymmetrical (असममित): no plane gives two equal halves, as in some sponges. Radial (त्रिज्यीय): any plane through the central axis gives equal halves; a starfish has five such planes. Bilateral (द्विपार्श्व): only one plane gives two mirror halves, as in insects, fishes, frogs, birds and humans.
In the figure : In the figure three kinds of body symmetry.
- Easy : A starfish looks the same from five directions: radial symmetry.
- Medium : A butterfly can be cut into two equal halves only along its midline: bilateral symmetry.
- Exam-type : Where does a jellyfish fit? Its body is a disc with parts around a central axis, so it is radially symmetrical.
Grades: protoplasmic → cellular → cell-tissue → tissue-organ → organ-system. Symmetry: asymmetrical, radial, bilateral.
Mixing up radial and bilateral. Radial has many planes; bilateral has only one.
Memory clue: "Star = many cuts, Human = one cut".
Spot the difference
| Symmetry | Planes giving equal halves | Example |
|---|---|---|
| Asymmetrical | None | Some sponges |
| Radial | Many | Starfish |
| Bilateral | One | Human, frog |
Practice questions
- 1In which kingdom are all multicellular animals placed?
- 2Name the five grades of organization.
- 3Which grade do Cnidaria show?
- 4What type of symmetry does a starfish show?
- 5What type of symmetry do humans show?
- 6Give one example each of protoplasmic and organ-system grade.
- 7Arrange in increasing complexity: Planaria, Amoeba, sponge, crab.
- 8Why can a starfish be cut into equal halves in many ways but a fish only in one?
Show answers (try them yourself first)
- 1Kingdom Animalia.
- 2Protoplasmic, cellular, cell-tissue, tissue-organ, organ-system.
- 3Cell-tissue grade.
- 4Radial.
- 5Bilateral.
- 6Amoeba; frog (or human).
- 7Amoeba, sponge, Planaria, crab.
- 8A starfish is radial with parts around a central axis; a fish is bilateral with a single midline plane.
Germ layers, body cavity and segmentation
A sandwich can have two slices or three. Early animal embryos build their bodies from two or three layers, like slices. Inside, some animals have an empty room in the body for organs; others do not.
Germ layers (भ्रूणस्तर) are the layers of the embryo; the coelom (देहगुहा) is the body cavity.
Rule 1: Germ layers — Diploblastic (द्विस्तरीय) animals have two layers, endoderm and ectoderm (all cnidarians). Triploblastic (त्रिस्तरीय) animals have a third layer, mesoderm, as well (most other animals).
In the figure : In the figure two or three germ layers.
- Easy : Hydra has an inner and an outer layer only: diploblastic.
- Medium : An earthworm has ectoderm, mesoderm and endoderm: triploblastic.
- Exam-type : Which layer is extra in triploblasts? The mesoderm, between the other two; muscles and many organs come from it.
Rule 2: Body cavity and segmentation — Eucoelomate (सत्य देहगुहायुक्त): true cavity lined by mesoderm (Annelida and all later phyla). Acoelomate (देहगुहाहीन): no cavity (Porifera, Cnidaria, Platyhelminthes). Pseudocoelomate (आभासी देहगुहायुक्त): cavity not formed from mesoderm (Aschelminthes). Segmentation (खंडीभवन): the body is divided into small similar units, as in the earthworm.
In the figure : In the figure three types of body cavity.
- Easy : Planaria has no body cavity: acoelomate.
- Medium : Ascaris has a cavity, but it is not lined by mesoderm: pseudocoelomate.
- Exam-type : An earthworm's body is made of repeated ring-like segments: it is segmented and also eucoelomate.
Diploblastic = 2 layers, triploblastic = 3. Coelom: acoelomate, pseudocoelomate, eucoelomate.
Thinking every cavity is a true coelom. A true coelom is lined by mesoderm.
Memory clue: "A for none, P for false, E for true".
Spot the difference
| Cavity type | Meaning | Example |
|---|---|---|
| Acoelomate | No cavity | Planaria |
| Pseudocoelomate | Not from mesoderm | Ascaris |
| Eucoelomate | From mesoderm | Earthworm |
Practice questions
- 1What are the germ layers of a diploblastic animal?
- 2Which germ layer is the third one in triploblastic animals?
- 3Is Hydra diploblastic or triploblastic?
- 4What is a coelom?
- 5Name an acoelomate and a pseudocoelomate animal.
- 6What is segmentation? Give an example.
- 7An animal has three germ layers and no body cavity. Name its likely phylum from this lesson.
- 8Why are cnidarians called acoelomate and diploblastic but not triploblastic?
Show answers (try them yourself first)
- 1Ectoderm and endoderm.
- 2Mesoderm.
- 3Diploblastic.
- 4The body cavity lined by mesoderm.
- 5Planaria (acoelomate); Ascaris (pseudocoelomate).
- 6Division of the body into repeated similar units, e.g. earthworm.
- 7Platyhelminthes.
- 8They have only two germ layers and no mesoderm, so no true cavity forms.
Non-chordate phyla
A school has many classes: nursery, primary, secondary. Non-chordate animals also have many "classes" called phyla, each with its own special features. Knowing one feature of each helps you recognise an animal quickly.
A phylum (संघ) is a large group of animals sharing basic body features.
Rule 1: Porifera to Aschelminthes — Porifera (sponges): pores (ostia and oscula), collar cells, mostly asymmetrical, sedentary, with spicules or spongin; Sycon, bath sponge. Cnidaria: polyp or medusa form, radial, diploblastic, stinging cnidoblasts on tentacles; Hydra, sea anemone, jellyfish, corals. Platyhelminthes: flat, mostly endoparasites, acoelomate, triploblastic, hermaphrodite; Planaria, liver fluke, tapeworm. Aschelminthes: thread-like, pseudocoelomate, unisexual; Ascaris, filarial worm.
In the figure : In the figure common non-chordate phyla with an example each.
- Easy : A sponge sits fixed on a rock and filters water through its pores.
- Medium : A tapeworm is flat, lives inside a host and is hermaphrodite: Platyhelminthes.
- Exam-type : Ascaris is thread-like, has separate sexes and a pseudocoelom, so it belongs to Aschelminthes, not to flatworms.
Rule 2: Annelida to Hemichordata — Annelida: segmented, eucoelomate, with setae or parapodia; earthworm, leech, Nereis. Arthropoda: jointed appendages, chitinous exoskeleton, the largest phylum; crab, spider, cockroach, butterfly. Mollusca: soft body with head, foot, visceral mass and a mantle that makes the shell; second largest phylum; snail, bivalve, octopus. Echinodermata: calcareous spines, only marine, radial adults, tube feet, power of regeneration; starfish, sea urchin. Hemichordata: proboscis, collar and trunk; notochord only in the proboscis; Balanoglossus links non-chordates and chordates.
- Easy : A crab has jointed legs and a hard chitin shell: Arthropoda.
- Medium : An octopus has a soft body and a mantle even though it has no outer shell: Mollusca.
- Exam-type : A starfish has tube feet and spines, lives only in the sea and regrows lost arms: Echinodermata.
Arthropoda is the largest phylum, Mollusca the second. Balanoglossus is a connecting link.
Putting leeches with flatworms. A leech is segmented, so it is an annelid.
Memory clue: "Jointed legs = Arthropoda, soft body + mantle = Mollusca, spiny star = Echinoderm".
Spot the difference
| Phylum | Key feature | Example |
|---|---|---|
| Porifera | Pores, sedentary | Sycon |
| Cnidaria | Cnidoblasts | Hydra |
| Annelida | Segmented | Earthworm |
| Arthropoda | Jointed legs | Cockroach |
Practice questions
- 1Which phylum has cnidoblasts?
- 2Which is the largest phylum of animals?
- 3What does the mantle of a mollusc make?
- 4Name the connecting link between non-chordates and chordates.
- 5Name one parasitic flatworm.
- 6Which phylum does a leech belong to and why?
- 7An animal is marine, has radial adults, tube feet and regenerates. Name its phylum.
- 8How are Platyhelminthes and Aschelminthes different?
Show answers (try them yourself first)
- 1Cnidaria.
- 2Arthropoda.
- 3The shell.
- 4Balanoglossus (Hemichordata).
- 5Liver fluke (or tapeworm).
- 6Annelida, because its body is segmented with a true coelom.
- 7Echinodermata.
- 8Platyhelminthes are flat, acoelomate and hermaphrodite; Aschelminthes are thread-like, pseudocoelomate and unisexual.
Chordates and the classes of vertebrates
A bus conductor knows who is a child, an adult or a senior citizen at a glance. In the same way, a student can say which class a vertebrate belongs to by checking its skin, breathing and body temperature.
Vertebrata (पृष्ठवंशी) have a backbone and are divided into six classes.
Rule 1: Chordates — At some stage chordates have a notochord, pharyngeal gill-slits, a single tubular dorsal nerve cord and a ventral heart. Urochordata: marine, with a tunic (Herdmania). Cephalochordata: small and fish-like with the notochord along the whole body (Amphioxus). Vertebrata: a vertebral column replaces the notochord; well-developed head and cranium; six classes.
- Easy : Herdmania lives in the sea and is covered by a tunic: Urochordata.
- Medium : Amphioxus keeps its notochord for life along the whole body: Cephalochordata.
- Exam-type : All vertebrates are chordates, but not all chordates are vertebrates, since Urochordata and Cephalochordata have no backbone.
Rule 2: Six classes of vertebrates — Cyclostomata: jaw-less, sucker mouth, ectoparasites (Petromyzon). Pisces: cold-blooded, spindle-shaped, fins, gills, scales (rohu, shark). Amphibia: aquatic larvae, moist skin, no claws (frog, toad). Reptilia: first truly terrestrial, creeping, dry scaly skin, claws (tortoise, lizard, snake). Aves: warm-blooded, wings, feathers, beak (pigeon, penguin). Mammalia: mammary glands, warm-blooded, hair or fur (elephant, human, dolphin, bat).
In the figure : In the figure the six classes of vertebrates.
- Easy : A pigeon has feathers, wings and a beak and is warm-blooded: Aves.
- Medium : A lizard has dry scaly skin and claws and creeps on land: Reptilia.
- Exam-type : A frog starts as a tadpole in water and has moist skin: Amphibia. A shark has gills and fins and is cold-blooded: Pisces.
Six classes: Cyclostomata, Pisces, Amphibia, Reptilia, Aves, Mammalia. Aves and Mammalia are warm-blooded.
Calling a dolphin a fish. It has mammary glands and lungs, so it is a mammal.
Memory clue: "Cyclostomes, Fish, Frogs, Reptiles, Birds, Mammals: C-P-A-R-A-M".
Spot the difference
| Class | Body temperature | Example |
|---|---|---|
| Pisces | Cold-blooded | Rohu |
| Amphibia | Cold-blooded | Frog |
| Reptilia | Cold-blooded | Snake |
| Aves | Warm-blooded | Pigeon |
| Mammalia | Warm-blooded | Human |
Practice questions
- 1Name the six classes of vertebrates.
- 2Which class has mammary glands?
- 3Which class has feathers?
- 4Which were the first truly terrestrial vertebrates?
- 5Name one example each of Urochordata and Cephalochordata.
- 6A cold-blooded animal has gills, fins and scales. Which class?
- 7Give one similarity and one difference between Aves and Mammalia.
- 8Why is it true that all vertebrates are chordates but not the reverse?
Show answers (try them yourself first)
- 1Cyclostomata, Pisces, Amphibia, Reptilia, Aves, Mammalia.
- 2Mammalia.
- 3Aves.
- 4Reptilia.
- 5Herdmania; Amphioxus.
- 6Pisces.
- 7Both are warm-blooded; Aves have feathers and a beak, Mammalia have hair and mammary glands.
- 8Vertebrates have a notochord at some stage, but some chordates (Urochordata, Cephalochordata) lack a backbone.
Quick revision
- Chordates have a notochord; non-chordates do not.
- Grades: protoplasmic, cellular, cell-tissue, tissue-organ, organ-system.
- Vertebrate classes: Cyclostomata, Pisces, Amphibia, Reptilia, Aves, Mammalia.
Quick revision
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