Lesson 3: Invertebrates - Characteristics, Classification and Examples, Apuntes de Biología

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LESSON 3:

INVERTEBRATES

Invertebrates all share one characteristic: They don´t have a backbone or an internal skeleton.Most invertebrates lay eggs.

They can be classified into six groups: ARTHROPODS MOLLUSCS SPONGES CNIDARIANS ECHINODERMS ANNELIDS

ARTHROPODS

This is the largest invertebrate group. All arthropods have an external skeleton called EXOSKELETON.They also have jointed legs that bend. Some arthropods have simple lungs. Others, like crabs and lobsters, absorb oxygen through their gills.They are oviparous. As the arthropod gets bigger, it sheds its exoskeleton and grows a new one. Arthropods are divided into four groups:

1. Insects
2. Arachnids
3. Crustaceans
4. Myriapods

Crustaceans have two body parts: an abdomen and a cephalothorax.Their exoskeleton is very hard, like a shell.They have two pairs of antennae and five pairs of legs. The front pair of legs has claws. Most crustaceans live in water. Examples of crustaceans are: crabs and shrimps(camarones). Myriapods have long, thin, segmented bodies. Thay have many pairs of legs and one pair of antennae. They are found in dark, damp places. Examples of myriapods are: centipedes and millipedes.

MOLLUSCS

Mollucs are soft-bodied invertebrates found on land and in water. Most of them have a hard shell.They absorb oxygen through gills and reproduce by laying eggs. Snails have a single shell and a single foot which they use to move. They have tentacles on their heads, and each tentacle has an eye at the end of it. They also use their tentacles to feel. Other molluscs, such as mussels, have shells that can open and close. They have a soft body but no head. Octopuses don´t have a shell. To move about, they expel jets of water and move their tentacle.

SPONGES

When you think of sponges, you probably picture the sponge in your kitchen. Believe it or not, your kitchen sponge may actually have come from a sea sponge!

• The polyp is related to the anemone, and consists of a stomach with a mouth at one end. Polyps live attached to rocks.They also have tentacles and mouth.But they differ from jellyfish that their mouth and tentacles are in the upper part of their body. Polyps make coral.

ECHINODERMS

 Echinoderm means echino(spiny) and derm means (skin).  They live in all parts of the ocean, but mostly on the sea floor.They have an internal structure called endoskeleton, and they are covered by hard, spiny skin.  Many echinoderms catch their food by grabbing it with their tiny tube feet. Starfish and sea urchins are examples of echinoderms.

 Echinoderms have their mouth at the centre of their bodies. They pull out their stomachs to digest the food.  They don´t have brain.  Some echinoderms such as the brittle stars eat catching food particles that float in the water Others, like the starfish capture their prey by bending their arms to push the food into their mouths. EXAMPLES:

• Starfish. They have a spiny body that usually divides into five equal parts.Their feet are what´s bellow them.The shape of the feet is like a tube so they can stick to a wall.
• Sea urchins have sharp spines covering its hard skeleton and protect it from attack. IMPORTANT FACT: When a starfish loses one of its legs, a new one will grow on its place

• Segmented worms The earthworm is a segmented worm. It isn´t a parasite. Annelids don´t have a protective body covering. Some annelids breathe through their skin(worms), but others have gills. Examples of annelids are leeches (sanguijuelas) and worms(gusanos).

ACTIVITIES

1.Classify these invertebrates and write a sentence to describe each one. a) b) c) d) Picture a) shows a/an …………………………… It has ……………………………………………………………………………… ………………… Picture b) shows a/an ……………………………

It has ……………………………………………………………………………… ………………… Picture c) shows a/an …………………………… It has ……………………………………………………………………………… ………………… Picture d) shows a/an …………………………… It has ……………………………………………………………………………… ………………… 2 .Classify Arthropods, mollucs, sponges, cnidarians,echinoderms and annelids in this chart: SPIDER, SEA ANEMONE, EARTHWORM, SEA CUCUMBER, STARFISH, SPONGE, OCTOPUS, ANT, CENTIPEDE, JELLYFISH. ARTHROPODS MOLLUSCS SPONGES CNIDARIANS ECHINODERMS ANNELIDS

3. Label the picture of this ant.

6. Invertebrates haven't got a ____________