Invertebrates are animals that do not have a backbone. Invertebrates are cold-blooded; their body temperature depends on the temperature of their environment. 

Some major groups of invertebrates include: 
Protozoans - Very primitive, simple animals like amoebas 
Some of the Metazoa - Porifera (sponges), jellyfish, corals, tapeworms, flukes, insects, arachnids, crustaceans, mollusks, echinoderms, etc.