Although all the vertebrates belong basically to one phylum (Phylum Chordata), the rest of the animal kingdom is classified into many phyla reflecting the enormous diversity found within this group of animals without backbones which we loosely call the Invertebrates.

Sponges
Sponges are also called Phylum Porifera. The sponge's body is a simple, asymmetrical sac with different specialised cells, but no specific tissues or organs. They feed by filtering food from the water through their pores into the body cavity.

Coelenterates
These include jellyfish, reef corals, sea anemones (Phylum Cnidaria or Coelenterata). They are water-dwelling, radially symmetrical animals which have stinging cells in the tentacles around the mouth. There is some specialisation of cells into tissues.

Worms
They include Flatworms, such as tapeworms (Phylum Platyhelminthes), Roundworms (Phylum Nematoda) and Segmented worms, such as Earthworms (Phylum Annelida). These are bilaterally symmetrical with a definite head. They are the simplest groups with organ systems. Most live in water but some live in soil, while others are parasites in host animals.

Molluscs
Molluscs include squid, bivalve shells, snails and slugs (Phylum Mollusca). The molluscs are mainly marine dwellers, though snails and slugs live on land. They are bilaterally symmetrical. They have a soft body, muscular foot, well-developed body systems and often secrete a hard shell.

Echinoderms
These are starfish, sea urchins and sea cucumbers (Phylum Echinodermata). These are all marine, radially symmetrical organisms with sucker-like tube feet and an internal lime skeleton.

Arthropods
These include crustaceans, spiders, centipedes and insects (Phylum Arthropoda). This is the largest animal phylum with more than a million species.

These have a hard external skeleton with jointed appendages (spiders have eight legs, insects have six). Body is usually segmented into head, thorax and abdomen. They have successfully colonised all habitats, marine, freshwater, and terrestrial.

• Crustaceans e.g. crab, crayfish, shrimp.
  
  
• Insects e.g. fly, ant, butterfly, dragonfly, mosquito.
  
  
• Arachnids e.g. spiders
  
  
• Myriapods e.g. centipedes and millipedes.