Animal Classification Class 10 Notes, Formulas, Questions | SSC

This chapter deals with the classification of the huge variety of animals around us. It traces the history of classification from Aristotle's artificial method to natural and evolutionary systems. It then explains the criteria of the new system: grade of organization, symmetry, germ layers, body cavity and segmentation. Finally it describes the characters and examples of each phylum of Kingdom Animalia and the six classes of vertebrates.

Summary

Living organisms show great diversity, and the estimated number of animal species on earth is about 7 million. Studying each species separately is impossible, so animals are placed in groups and sub-groups on the basis of similarities and differences; this is animal classification. Classification makes study convenient, lets the study of a few animals explain a whole group, gives an idea of evolution, helps accurate identification, and helps us understand relationships, habitats, roles in nature and adaptations of animals.

Aristotle was the first to classify animals, using body size, habits and habitats. His method is called the artificial method, which was also followed by Theophrastus, Pliny, John Ray and Linnaeus. Later the natural system used body organization, types of cells, chromosomes and biochemical properties. Classification based on evolution was used by Dobzhansky and Meyer, and recently Carl Woese also proposed a classification.

In the traditional method, animals were divided into non-chordates and chordates depending on the presence or absence of a notochord. Non-chordates have no notochord, no pharyngeal gill slits, a solid paired ventral nerve cord (if present) and a dorsal heart (if present); they were placed in ten phyla from Protozoa to Hemichordata. Chordates have a notochord, pharyngeal gill slits or lungs, a hollow dorsal nerve cord and a ventral heart. The notochord is a long rod-like supporting structure on the dorsal side that keeps nerve tissue separate from the rest of the body.

In the new system, following Robert Whittaker's five kingdom classification, all multicellular animals are placed in Kingdom Animalia. The criteria are grade of organization, body symmetry, germ layers, body cavity and segmentation. Grades of organization are protoplasmic (unicellular), cellular (Porifera, no tissues), cell-tissue (Cnidaria), tissue-organ (flatworms) and organ-system (crab, frog, human). Bodies may be asymmetrical (some sponges), radially symmetrical (starfish) or bilaterally symmetrical (insects, fish, frog, birds, human).

Animals with only ectoderm and endoderm are diploblastic (all cnidarians), while those that also have mesoderm are triploblastic. The coelom is the cavity between the body wall and internal organs. Annelida and all later phyla are eucoelomates with a true coelom; Porifera, Cnidaria and Platyhelminthes are acoelomates; Aschelminthes are pseudocoelomates. A body divided into small similar units called segments is a segmented body, as in the earthworm.

Using these criteria Kingdom Animalia is divided into ten phyla. Porifera are pore-bearing, mostly marine, sedentary sponges with collar cells and a skeleton of spicules or spongin. Cnidaria are radially symmetrical, diploblastic, polyp or medusa forms with cnidoblast-bearing tentacles. Platyhelminthes are flat, acoelomate, triploblastic, mostly endoparasitic hermaphrodites. Aschelminthes are thread-like, pseudocoelomate, unsegmented, unisexual round worms covered with a tough cuticle.

Annelida are metamerically segmented eucoelomates moving by setae, parapodia or suckers. Arthropoda, the largest phylum, have jointed appendages and a chitinous exoskeleton. Mollusca, the second largest phylum, are soft-bodied with head, foot and visceral mass, and a mantle that secretes a calcareous shell. Echinodermata are marine animals with calcareous spines, tube feet, radial symmetry in adults and bilateral symmetry in larvae, and good regeneration. Hemichordata (acorn worms) have a body of proboscis, collar and trunk with a notochord only in the proboscis; Balanoglossus is a connecting link between non-chordates and chordates.

Phylum Chordata has a notochord and pharyngeal gill slits at least at some developmental stage, a single dorsal tubular nerve cord and a ventral heart. It has three subphyla: Urochordata (notochord only in the tail of the larva, body covered by a tunic, e.g. Herdmania), Cephalochordata (notochord along the whole body, e.g. Amphioxus) and Vertebrata, where the notochord is replaced by a vertebral column and the brain is protected by a cranium. Vertebrata has six classes: Cyclostomata, Pisces, Amphibia, Reptilia, Aves and Mammalia.

Mind map

Grades of body organization from simple to complex

Important definitions

Key points to remember

• About 7 million animal species are estimated to exist on earth.
• Aristotle was the first to classify animals; his method is called the artificial method.
• Traditional division: non-chordates (ten phyla) and chordates, on the basis of the notochord.
• New system follows Robert Whittaker's five kingdom classification; all multicellular animals are in Kingdom Animalia.
• Criteria of the new system: grade of organization, symmetry, germ layers, coelom and segmentation.
• Cnidarians are diploblastic; most other animals are triploblastic.
• Porifera, Cnidaria and Platyhelminthes are acoelomate; Aschelminthes are pseudocoelomate; Annelida onwards are eucoelomate.
• Collar cells are special cells of sponges; cnidoblasts are stinging cells of cnidarians.
• Arthropoda is the largest phylum; Mollusca is the second largest.
• Octopus is the most clever non-chordate and can change its colour.
• Echinoderms are radially symmetrical as adults but bilaterally symmetrical as larvae.
• Balanoglossus is the connecting link between non-chordates and chordates.
• In vertebrates the notochord is replaced by a vertebral column and the brain is protected by a cranium.
• Reptiles are the first true terrestrial animals; birds and mammals are warm-blooded.

Solved examples

Example 1. Classify the shark up to class.

Step 1: It is multicellular without a cell wall, so Kingdom Animalia. Step 2: It has a notochord stage, pharyngeal gill slits and a dorsal nerve cord, so Phylum Chordata. Step 3: Notochord is replaced by a vertebral column and the brain is enclosed in a cranium, so Subphylum Vertebrata. Step 4: It is aquatic, has fins, breathes by gills and has scales, so Class Pisces.

Example 2. Identify the phylum: I am multicellular but there are no tissues in my body.

A multicellular body without tissues shows cellular grade organization. This grade is found in Phylum Porifera (sponges). Answer: Porifera.

Example 3. Identify me: I live in your small intestine and my thread-like body has a pseudocoelom.

A thread-like or cylindrical body with a pseudocoelom is the feature of round worms. The worm living in the human intestine is Ascaris. Phylum: Aschelminthes.

Example 4. To which phylum does the cockroach belong? Justify.

The cockroach has jointed legs (jointed appendages), a body covered by a chitinous exoskeleton, a segmented, bilaterally symmetrical, triploblastic and eucoelomate body, and separate sexes. These are the characters of Phylum Arthropoda, so the cockroach is an arthropod.

Example 5. Classify the bat up to class.

Kingdom Animalia (multicellular, no cell wall); Phylum Chordata (notochord, gill slits at some stage, dorsal hollow nerve cord); Subphylum Vertebrata (vertebral column, cranium); Class Mammalia, because it has mammary glands, hair on the body and is warm-blooded, even though it flies with a patagium.

Important questions and answers

Q1. Which special cells are present in the body of sponges?

Collar cells.

Q2. Which of these shows bilateral symmetry: starfish, jellyfish, earthworm, sponge?

Earthworm.

Q3. Which of these can regenerate a broken body part: cockroach, frog, sparrow, starfish?

Starfish.

Q4. Give reason: Though the tortoise lives on land as well as in water, it is not included in Amphibia.

The tortoise has dry scaly skin, a neck, and digits with claws, and it is cold-blooded and creeps on land. These are reptilian characters. Amphibians have moist skin without scales or claws and their larvae are strictly aquatic. So the tortoise belongs to Class Reptilia, not Amphibia.

Q5. Give reason: Our body gets irritated if it touches a jellyfish.

The jellyfish is a cnidarian. Its tentacles bear cnidoblasts which inject toxin into anything they touch, for capturing prey and for protection. This toxin causes irritation of our skin.

Q6. Give reason: All vertebrates are chordates but all chordates are not vertebrates.

All vertebrates have a notochord at some developmental stage, gill slits, a dorsal nerve cord and a ventral heart, so they are chordates. But Urochordata and Cephalochordata also have these chordate characters and still keep the notochord without a vertebral column. Hence not all chordates are vertebrates.

Q7. Give reason: Balanoglossus is a connecting link between non-chordates and chordates.

Balanoglossus is a hemichordate that shows characters of both groups. It has a notochord only in the proboscis region and has pharyngeal gill slits, which are chordate features, while its other features are like non-chordates. Hence, from the viewpoint of evolution, it is a connecting link.

Q8. Give reason: Body temperature of reptiles is not constant.

Reptiles are cold-blooded (poikilotherms). They cannot regulate their body temperature, so it changes with the temperature of the surroundings.

Q9. Write four distinguishing characters of Phylum Echinodermata.

1. Calcareous spines on the body. 2. Found only in the ocean. 3. Radially symmetrical adults but bilaterally symmetrical larvae. 4. Locomotion and capture of prey by tube feet; they also have good regeneration ability. Examples: starfish, sea urchin.

Q10. Distinguish between butterfly and bat.

Butterfly: 1. Non-chordate of Phylum Arthropoda. 2. No notochord or vertebral column. 3. Jointed appendages and a chitinous exoskeleton. 4. No mammary glands. Bat: 1. Chordate of Class Mammalia. 2. Vertebral column and cranium present. 3. Exoskeleton in the form of hair; it flies with a skin fold called the patagium. 4. Mammary glands present and it is warm-blooded.

Q11. What is the difference between grade of organization and symmetry?

Grade of organization tells how cells are organized in the body: only cells (Porifera), cells and tissues (Cnidaria), tissues and organs (flatworms) or organ systems (frog, human). Symmetry tells whether an imaginary plane can divide the body into equal halves: asymmetrical (some sponges), radial (starfish) or bilateral (insects, human). One describes internal level of organization, the other the body's outer plan.

Q12. Write in brief about progressive changes in animal classification.

Aristotle first classified animals by body size, habits and habitat (artificial method), followed by Theophrastus, Pliny, John Ray and Linnaeus. Then the natural system used body organization, cell types, chromosomes and biochemical properties. Later Dobzhansky and Meyer used evolution-based classification, and Carl Woese also proposed a system. Today animals are classified under Whittaker's five kingdom system using organization, symmetry, germ layers, coelom and segmentation.

Exam tips

• Learn one or two examples for every phylum and class; 'Identify me' questions depend on them.
• Remember the coelom pattern: acoelomate up to Platyhelminthes, pseudocoelomate Aschelminthes, eucoelomate from Annelida.
• For classification questions write Kingdom, Phylum, Subphylum and Class in order with one reason for each.
• Practise labelled diagrams of Hydra, Planaria, earthworm, cockroach, starfish and frog.
• Do not confuse habitat with class: whale is a mammal, crocodile is a reptile.

FAQs

Who proposed the five kingdom classification?

Robert Whittaker. In it, all multicellular animals are placed in Kingdom Animalia.

Which is the largest phylum of animals?

Arthropoda has the highest number of animals; Mollusca is the second largest.

Why is a whale not a fish?

The whale has mammary glands and is warm-blooded, so it belongs to Class Mammalia even though it lives in the ocean.

How many classes are there in subphylum Vertebrata?

Six: Cyclostomata, Pisces, Amphibia, Reptilia, Aves and Mammalia.

These notes were prepared with AI help from the Balbharati Science and Technology Part 2 textbook (2026-27) and checked against it.