Most Animals on Earth Have No Backbone at All
Learn the basis of animal classification, tell invertebrates from vertebrates, and work through Porifera, Coelenterata, Platyhelminthes, Aschelminthes, Annelida, Mollusca, Arthropoda and Echinodermata.
How many animals actually have a backbone?
Very few. The great majority of animal species are invertebrates — sponges, jellyfish, worms, snails, insects and starfish — and only a small fraction have a backbone at all.
So the animals most familiar to us are the exception, not the rule. This page covers everything in the ICSE Class 7 Biology chapter's first part: the basis of animal classification, and the eight invertebrate phyla.
So the animals most familiar to us are the exception, not the rule. This page covers everything in the ICSE Class 7 Biology chapter's first part: the basis of animal classification, and the eight invertebrate phyla.
What is animal classification based on?
Animals are sorted on four main characteristics.
- Presence or absence of a backbone — this is the first and biggest division. Invertebrates have no backbone or internal skeleton; vertebrates have a vertebral column made of vertebrae.
- Body symmetry — asymmetrical with no regular shape (sponges), radial with parts arranged around a central axis (jellyfish, starfish), or bilateral with identical left and right halves (worms, insects, humans).
- Level of organisation — whether the body is a loose collection of cells, or has tissues, organs and organ systems.
- Body cavity (coelom) — whether a true cavity is present between the gut and the body wall.
An invertebrate may still be hard on the outside. A cockroach and a prawn both have tough exoskeletons, yet neither has a backbone — the skeleton is outside, not inside.
That is the misconception worth clearing early. "No backbone" does not mean "soft". It means no internal vertebral column, and an animal can be well armoured without one.
- Presence or absence of a backbone — this is the first and biggest division. Invertebrates have no backbone or internal skeleton; vertebrates have a vertebral column made of vertebrae.
- Body symmetry — asymmetrical with no regular shape (sponges), radial with parts arranged around a central axis (jellyfish, starfish), or bilateral with identical left and right halves (worms, insects, humans).
- Level of organisation — whether the body is a loose collection of cells, or has tissues, organs and organ systems.
- Body cavity (coelom) — whether a true cavity is present between the gut and the body wall.
An invertebrate may still be hard on the outside. A cockroach and a prawn both have tough exoskeletons, yet neither has a backbone — the skeleton is outside, not inside.
That is the misconception worth clearing early. "No backbone" does not mean "soft". It means no internal vertebral column, and an animal can be well armoured without one.
What are the features of sponges, jellyfish and flatworms?
These are the three simplest phyla.
Porifera — the sponges. The body is asymmetrical, with many tiny pores called ostia and one large opening, the osculum. Water flows in through the pores, bringing food and oxygen. There is a cellular level of organisation, with no tissues, organs or organ systems, and the body is supported by a skeleton of spicules. Mostly marine and fixed to one spot. Examples: Sycon, Euplectella (Venus flower basket), bath sponge.
Coelenterata (Cnidaria) — the jellyfish and their relatives. The body is radially symmetrical with a single central body cavity and a mouth ringed by tentacles. They have a tissue level of organisation, and special stinging cells called cnidoblasts for capturing prey and defence. All aquatic, mostly marine. Examples: Hydra, jellyfish, sea anemone, corals.
Platyhelminthes — the flatworms. The body is flat, ribbon-like and bilaterally symmetrical, with an organ level of organisation. There is no body cavity, and most are parasites with hooks and suckers to hold on to the host. Examples: tapeworm, liver fluke, Planaria.
The corals that build reefs off the Indian coast are colonies of coelenterates, and tapeworm is a parasite doctors warn about in undercooked food.
The progression is the point of this section. Porifera has cells only, Coelenterata reaches tissues, Platyhelminthes reaches organs — so the three phyla in order show the body becoming steadily better organised.
Porifera — the sponges. The body is asymmetrical, with many tiny pores called ostia and one large opening, the osculum. Water flows in through the pores, bringing food and oxygen. There is a cellular level of organisation, with no tissues, organs or organ systems, and the body is supported by a skeleton of spicules. Mostly marine and fixed to one spot. Examples: Sycon, Euplectella (Venus flower basket), bath sponge.
Coelenterata (Cnidaria) — the jellyfish and their relatives. The body is radially symmetrical with a single central body cavity and a mouth ringed by tentacles. They have a tissue level of organisation, and special stinging cells called cnidoblasts for capturing prey and defence. All aquatic, mostly marine. Examples: Hydra, jellyfish, sea anemone, corals.
Platyhelminthes — the flatworms. The body is flat, ribbon-like and bilaterally symmetrical, with an organ level of organisation. There is no body cavity, and most are parasites with hooks and suckers to hold on to the host. Examples: tapeworm, liver fluke, Planaria.
The corals that build reefs off the Indian coast are colonies of coelenterates, and tapeworm is a parasite doctors warn about in undercooked food.
The progression is the point of this section. Porifera has cells only, Coelenterata reaches tissues, Platyhelminthes reaches organs — so the three phyla in order show the body becoming steadily better organised.
What are the features of roundworms, earthworms and snails?
These three phyla are a further step up in organisation.
Aschelminthes (Nemathelminthes) — the roundworms. The body is long, cylindrical and unsegmented, tapering at both ends, and bilaterally symmetrical. It has a false body cavity, and a complete digestive tract with a mouth and anus. Many are parasites of humans, animals and plants. Examples: Ascaris (roundworm), hookworm, filarial worm.
Annelida — the segmented worms. The body is long, cylindrical and divided into many ring-like segments, with a true body cavity (coelom) — the first phylum to have one. They move using bristles called setae, and have an organ-system level of organisation. Examples: earthworm, leech, Nereis.
Mollusca — the soft-bodied animals. The body is soft and unsegmented, usually protected by a hard calcareous shell, with a muscular foot for movement and a rasping radula for feeding. It is the second largest phylum. Examples: snail, Pila, octopus, mussel, oyster.
Earthworms turned up by a spade in a monsoon-wet garden are annelids, and they are called the farmer's friend because their burrowing aerates and enriches the soil.
The feature that marks the real advance here is segmentation with a true coelom in Annelida. An octopus is a mollusc despite having no external shell — the shell is reduced internally, which shows that the phylum is defined by body plan and not simply by whether a shell is visible.
Aschelminthes (Nemathelminthes) — the roundworms. The body is long, cylindrical and unsegmented, tapering at both ends, and bilaterally symmetrical. It has a false body cavity, and a complete digestive tract with a mouth and anus. Many are parasites of humans, animals and plants. Examples: Ascaris (roundworm), hookworm, filarial worm.
Annelida — the segmented worms. The body is long, cylindrical and divided into many ring-like segments, with a true body cavity (coelom) — the first phylum to have one. They move using bristles called setae, and have an organ-system level of organisation. Examples: earthworm, leech, Nereis.
Mollusca — the soft-bodied animals. The body is soft and unsegmented, usually protected by a hard calcareous shell, with a muscular foot for movement and a rasping radula for feeding. It is the second largest phylum. Examples: snail, Pila, octopus, mussel, oyster.
Earthworms turned up by a spade in a monsoon-wet garden are annelids, and they are called the farmer's friend because their burrowing aerates and enriches the soil.
The feature that marks the real advance here is segmentation with a true coelom in Annelida. An octopus is a mollusc despite having no external shell — the shell is reduced internally, which shows that the phylum is defined by body plan and not simply by whether a shell is visible.
Why is Arthropoda the largest phylum in the animal kingdom?
Because its body plan is extraordinarily adaptable, letting arthropods live almost anywhere on Earth.
Arthropoda — the name means jointed legs. The features are:
- Jointed legs and a segmented body, usually divided into head, thorax and abdomen.
- A hard chitinous exoskeleton, shed periodically by moulting so the animal can grow.
- Bilateral symmetry and an organ-system level of organisation.
- Often compound eyes and jointed appendages adapted for many uses.
Examples: cockroach, butterfly, prawn, crab, spider, scorpion, centipede, millipede.
The reasons for its size are worth listing, because the question asks why:
- The exoskeleton protects the body and prevents water loss, allowing life in dry places.
- Jointed legs allow walking, running, swimming, burrowing and jumping.
- Many have wings, so they can fly and spread widely.
- Their small size lets them occupy habitats larger animals cannot.
Echinodermata — the spiny-skinned animals. All are marine. The body shows radial symmetry in the adult, has a spiny skin with a calcareous endoskeleton, a unique water vascular system with tube feet for movement and feeding, and a remarkable power of regeneration. Examples: starfish, sea urchin, sea cucumber.
Cockroaches surviving in kitchens across every Indian climate make the arthropod case better than any list — the same body plan works in a house, a field, a pond and a desert.
The boundary case in Echinodermata is its symmetry: the larva is bilateral while the adult is radial, which is why classification questions specify the adult.
Arthropoda — the name means jointed legs. The features are:
- Jointed legs and a segmented body, usually divided into head, thorax and abdomen.
- A hard chitinous exoskeleton, shed periodically by moulting so the animal can grow.
- Bilateral symmetry and an organ-system level of organisation.
- Often compound eyes and jointed appendages adapted for many uses.
Examples: cockroach, butterfly, prawn, crab, spider, scorpion, centipede, millipede.
The reasons for its size are worth listing, because the question asks why:
- The exoskeleton protects the body and prevents water loss, allowing life in dry places.
- Jointed legs allow walking, running, swimming, burrowing and jumping.
- Many have wings, so they can fly and spread widely.
- Their small size lets them occupy habitats larger animals cannot.
Echinodermata — the spiny-skinned animals. All are marine. The body shows radial symmetry in the adult, has a spiny skin with a calcareous endoskeleton, a unique water vascular system with tube feet for movement and feeding, and a remarkable power of regeneration. Examples: starfish, sea urchin, sea cucumber.
Cockroaches surviving in kitchens across every Indian climate make the arthropod case better than any list — the same body plan works in a house, a field, a pond and a desert.
The boundary case in Echinodermata is its symmetry: the larva is bilateral while the adult is radial, which is why classification questions specify the adult.
Exam tip
Exam tip: giving the symmetry and two examples
Phylum questions follow a fixed shape, and answering in that shape collects every mark.
Give symmetry, body plan, one special feature and two examples for each phylum. Coelenterata: radially symmetrical, single body cavity, stinging cnidoblasts — Hydra and jellyfish.
Name the special structure that belongs to each phylum, because it is usually the distinguishing mark: ostia and osculum in Porifera, cnidoblasts in Coelenterata, setae in Annelida, radula and foot in Mollusca, jointed legs in Arthropoda, and the water vascular system in Echinodermata.
Use two examples, and prefer the standard ones. Ascaris and hookworm for Aschelminthes; earthworm and leech for Annelida.
When asked why Arthropoda is the largest, give at least three reasons — exoskeleton, jointed legs, wings — not just the statement.
And specify adult when writing about echinoderm symmetry.
Give symmetry, body plan, one special feature and two examples for each phylum. Coelenterata: radially symmetrical, single body cavity, stinging cnidoblasts — Hydra and jellyfish.
Name the special structure that belongs to each phylum, because it is usually the distinguishing mark: ostia and osculum in Porifera, cnidoblasts in Coelenterata, setae in Annelida, radula and foot in Mollusca, jointed legs in Arthropoda, and the water vascular system in Echinodermata.
Use two examples, and prefer the standard ones. Ascaris and hookworm for Aschelminthes; earthworm and leech for Annelida.
When asked why Arthropoda is the largest, give at least three reasons — exoskeleton, jointed legs, wings — not just the statement.
And specify adult when writing about echinoderm symmetry.
Did you know
Why must an insect shed its skin to grow bigger?
Because its skeleton is on the outside, and a hard case cannot stretch.
An arthropod's chitinous exoskeleton protects it and keeps water in, but it sets a fixed size. When the animal has grown to fill it, the only way forward is to split the old covering and climb out — a process called moulting.
The new exoskeleton underneath is soft at first, so the animal expands quickly before it hardens. That is also the most dangerous time in its life, which is why a freshly moulted crab or insect stays hidden until its armour sets.
An arthropod's chitinous exoskeleton protects it and keeps water in, but it sets a fixed size. When the animal has grown to fill it, the only way forward is to split the old covering and climb out — a process called moulting.
The new exoskeleton underneath is soft at first, so the animal expands quickly before it hardens. That is also the most dangerous time in its life, which is why a freshly moulted crab or insect stays hidden until its armour sets.
Key takeaways
Invertebrate phyla: quick revision
- Animals are classified by backbone, symmetry, level of organisation and body cavity; invertebrates lack an internal vertebral column but may still have a hard exoskeleton.
- Porifera: asymmetrical, ostia and osculum, cellular level — Sycon, bath sponge.
- Coelenterata: radial, single body cavity, cnidoblasts, tissue level — Hydra, jellyfish.
- Platyhelminthes: flat, bilateral, no body cavity, mostly parasitic — tapeworm, liver fluke.
- Aschelminthes: cylindrical and unsegmented with a false coelom — Ascaris, hookworm. Annelida: segmented with a true coelom and setae — earthworm, leech. Mollusca: soft, unsegmented, shell, foot and radula — snail, octopus.
- Arthropoda is the largest phylum thanks to its exoskeleton, jointed legs and wings; Echinodermata is marine with a water vascular system, and its adult is radial while its larva is bilateral.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Porifera: asymmetrical, ostia and osculum, cellular level — Sycon, bath sponge.
- Coelenterata: radial, single body cavity, cnidoblasts, tissue level — Hydra, jellyfish.
- Platyhelminthes: flat, bilateral, no body cavity, mostly parasitic — tapeworm, liver fluke.
- Aschelminthes: cylindrical and unsegmented with a false coelom — Ascaris, hookworm. Annelida: segmented with a true coelom and setae — earthworm, leech. Mollusca: soft, unsegmented, shell, foot and radula — snail, octopus.
- Arthropoda is the largest phylum thanks to its exoskeleton, jointed legs and wings; Echinodermata is marine with a water vascular system, and its adult is radial while its larva is bilateral.
You will remember all of this far better after answering five questions on it than after reading it twice.