Invertebrates are animals without backbones. They include annelids, nematodes, mollusks and arthropods. 

Arthropods are an important phylum of invertebrates. They have: 

• a hard exoskeleton (their skeleton is on the outside rather than on the inside) 
• a segmented body (their body has different sections) 
• jointed legs 

Arthropods can be divided into different groups depending on how many legs they have. 

Other invertebrate groups include: 

• annelids 
• nematodes 
• mollusks 

Unlike arthropods, these invertebrates do not have jointed legs. 

Annelids 

Annelids are segmented worms with soft bodies. Most of the worm’s body consists of identical segments, giving it a ‘ringed’ appearance.  

Nematodes 

Nematodes are tiny, thin worms without segments. Many species are free-living but others are parasites that cause disease. For example, hookworms can infect humans.

Molluscs 

Mollusks usually have a broad muscular foot and may also have a shell. Examples of mollusks include snails, slugs, squids and octopuses.