UNIT 10: INVERTEBRATES.
1. Invertebrate characteristics.
The group of animals called invertebrates is very varied; however, they have some things in common.
EXOSKELETON
Invertebrates do not have a backbone, but many of them have an external skeleton or exoskeleton. The exoskeleton
protects the animal. It is hard and rigid, so the animal cannot grow.
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When the animal grows, it eventually becomes too big for its exoskeleton.
The exoskeleton splits, and the animal comes out.
It already has a new skeleton. Initially the new skeleton is very soft, but it gradually hardens.
This process of getting a new exoskeleton is called moulting. It allows the animal to grow.
METAMORPHOSIS
Some invertebrates go through a big change called metamorphosis.
A good example of metamorphosis is when a caterpillar turns into a butterfly: the caterpillar transforms into a pupa
or chrysalis. Inside the pupa, the caterpillar turns into a butterfly. Finally, the adult butterfly comes out.
SYMMETRY
Most invertebrates are symmetrical. There are two types of symmetry:
Bilateral symmetry: you can draw a line down the centre of the animal to divide it into two identical halves. The
right half is a mirror image of the left half.
Radial symmetry: if there are lots of ways of dividing the animal into two identical halves. They also look the same if
you rotate them (like a starfish)
2. Porifera.
Porifera or sponges are very simple organisms that live in the sea.
What do they look like?
 Sponges are not symmetrical. They have many different shapes.
 Their bodies have many small holes or pores and channels. That is why they are called porifera.
Where do they live?
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Most of them live in marine ecosystems in clear water.
They are fixed to the sea floor.
What do they eat?
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Sponges eat bacteria and other small pieces of animals and plants in the water.
They have no stomach.
They absorb food through their whole bodies. Water flows in though the pores and the sponge
filters out small pieces of food from the water.
How do they reproduce?
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Sponges can reproduce sexually and asexually.
Sponges are hermaphrodites. This means that all sponges have both male and female sex organs.
They reproduce sexually by releasing sperm and eggs into the water.
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If part of the sponge’s body breaks off, it can grow into a new sponge. This is a type of asexual
reproduction
3. Cnidarians.
Cnidarians are aquatic animals and most of them live in the sea. Corals, jellyfish, sea anemones and hydra
are all cnidarian.
What do they look like?
 Cnidarians have radial symmetry.
 They have tentacles that contain stinging cells.
 They have two basic body forms: an umbrella shape or swimming medusa (jellyfish). They float
freely. And a bag shape or sessile polyps (sea anemones). They live fixed to the ground
Where do they live?
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Cnidarians usually live in marine ecosystems.
A few cnidarians live in freshwater ecosystems.
What do they eat?
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They are carnivores. They eat zooplankton.
They use their stinging cells to poison their prey (food).
Then they use their tentacles to move food into their mouths.
How do they reproduce?
Cnidarians can reproduce sexually (by releasing sperm and eggs into the water), and also asexually.
4. Worms.
The word “worms” is used to describe several groups of animals. The main types are flatworms
(Platyhelminthes), roundworms (Nematodes) and segmented worms (Annelids).
What do they look like?
 Worms are simple animals with long, thin bodies.
 They do not have a skeleton and they have no legs.
 They have bilateral symmetry.
PLATYHELMINTES (FLATWORMS). Example: tapeworm
Where do they live?
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Flatworms need to live in wet environments in order to breathe.
Many flatworms live in aquatic ecosystems.
Others are parasites that live inside larger animals.
What do they eat?
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Some flatworms are carnivores that eat small invertebrates.
Others are scavengers that eat pieces of dead plants and animals.
How do they reproduce?
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Flatworms are hermaphrodites (they have both male and female sex organs) that reproduce
sexually. They can fertilise themselves.
NEMATODES (ROUNDWORMS) Example: intestinal worm
Where do they live?
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Roundworms live in aquatic and terrestrial ecosystems. They often live in soil or mud.
Some roundworms are parasites- they live inside larger animals or in plants.
What do they eat?
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Many roundworms are scavengers that eat pieces of dead plants and animals.
Parasitic roundworms eat animal blood or plant sap.
How do they reproduce?
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Most roundworms species have male and females, but others are hermaphrodites.
They reproduce sexually.
ANNELIDS (SEGMENTED WORMS) Example: earthworm, leech.
Where do they live?
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Many segmented worms live in aquatic ecosystems.
Others, including earthworms, live in damp soil.
What do they eat?
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Most earthworms are scavengers that eat pieces of dead plants and animals.
Leeches are parasites that eat animal blood.
How do they reproduce?
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Segmented worms are hermaphrodites that reproduce sexually.
Most of them also reproduce asexually- part of a worm can grow into a new worm.
5. Molluscs.
There are lots of different kinds of molluscs, including snails, squid, octopuses and clams.
 Most molluscs have an exoskeleton called a shell.
 Most molluscs have bilateral symmetry when they are young. When their shells grow, some
molluscs lose their symmetry.
 We will look at three types of molluscs: bivalves (clams, cockles and mussels. Their shells have two
valves), gastropods (snails, sea snails and slugs. They have spiral-shaped shell with a single valve.
Exception: slugs have no shell) and cephalopods (squid, cuttlefish and octopi. They have tentacles,
but no shell).
What do they look like?
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They have a soft body divided into three parts:
- Head which contains sensorial organs and the mouth
- Body mass with the main organs
- Muscular foot to move about.
The body is covered by a fine membrane, the mantle. This produces a protective shell. The shell is
made up of one or two valves. Some species, such as octopi and slugs, have no shell. Others, such as
cuttlefish and squid, have an internal shell.
Where do they live?
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Most of them are aquatic animals: they live in the sea or in fresh water.
Some of them are terrestrial animals (snails, slugs) and they live in damp soil.
What do they eat?
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Molluscs that live in aquatic ecosystems are mainly carnivores. They eat small invertebrates or fish
(gastropods). Cephalopods catch animals with their arms or tentacles. Bivalves eat small pieces of
plants and animals in the water.
Most slugs and snails are herbivores.
How do they reproduce?
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Gastropods and Bivalves are hermaphrodites. Cephalopods are males and females.
They reproduce sexually.
6. Arthropods.
Arthropods are the largest (about 1 million different species), most varied group of living things: more than
one million species. That’s about 83% of all the species that we have found on the Earth.
They live everywhere on the planet: in terrestrial, marine and freshwater ecosystems.
What is an arthropod body like?
The main characteristics of arthropod bodies are:
 A segmented body covered by a thick cuticle that acts like an external skeleton or exoskeleton.
 A body divided into three parts: head, thorax and abdomen.
 In some arthropods, the head and thorax are joined to form a cephalothorax. The antennae, eyes
and mouth are in the head. The sensorial organs are well developed. The eyes can be simple: ocelli,
or compound.
 Bilateral symmetry.
 Jointed appendages: legs, antennae, wings in insects. The number of legs varies.
Arthropods functions
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Nutrition: Arthropods can be carnivorous, herbivorous or scavengers.
Respiration: they breathe through trachea (terrestrial arthropods) or gills (aquatic arthropods).
Reproduction: most have male and female sexes which are distinguishable. They are oviparous.
Fertilisation is internal. Some hatch as larvae and undergo metamorphosis.
As they grow, arthropods shed the old exoskeleton and grow a new one. This is called moulting.
Moulting takes place various times throughout an arthropod’s lifetime. In other words, arthropod
growth is discontinuous.
How many groups are there?
Groups
Crustaceans
Myriapods
Arachnids
Insects
Examples
Body/Appendages
Habitat
Lobster, crab
Centipede, scolopendra
Spider, scorpion
Butterfly, ant, bee, wasp
Cephalothorax and abdomen, 10 legs, 4 antennae
Head and trunk, many legs, 2 antennae
Cephalothorax and abdomen, 8 legs
Head, thorax and abdomen, 6 legs, 2 antennae, 2
or 4 or no wings
aquatic
terrestrial
terrestrial
Terrestrial,
some aquatic
7. Echinoderms
Echinoderm means “spiny skin”- echinoderms have skin that is covered with spines. They live on the sea bed.
Some live fixed to a surface, but others move slowly about. Examples: sea urchins, starfish and sea
cucumbers.
What is the body like?
The main characteristics of echinoderms are:
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Radial symmetry in adults, bilateral symmetry in larvae.
Body shape: rounded (sea urchin), cylindrical (sea cucumber) or like a star (starfish).
An internal skeleton made up of plaques just beneath their skin.
No separate head, but there is a mouth on the underside.
Echinoderms can move using tube-feet: the ambulacral apparatus, a series of internal tubes filled with
water. The tubes form ambulacral feet with suckers.
Echinoderms functions
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Respiration. Most echinoderms breathe through their skin, using the ambulacral apparatus. Some have
simple gills.
Nutrition. They are carnivorous and feed mainly on small crustaceans and molluscs.
Reproduction. Most echinoderms have male and female sexes, but some are hermaphrodite.
Fertilisation is external. The larvae can swim and undergo metamorphosis to change into adults.
How many groups are there?
There are five groups of Echinoderms: Echinoidea, Stelleroidea, Crinoidea, Holothuroidea and Ophiuroidea.
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Echinoidea: Sea urchins (round with many long spines).
Stelleroidea: Starfish (most adult starfish have five arms; some species have more arms, but they all have
radial symmetry).
Crinoidea: Sea lilies.
Holothuroidea: Sea cucumbers (cylindrical body shape)
Ophiuroidea: ophiura.