Animals can be classified into two main groups: vertebrates and invertebrates. The primary difference between the two is that vertebrates possess a backbone and an internal skeleton, while invertebrates do not have a backbone. Here are some key characteristics of each group:

Invertebrates

• Animals without spines
• Simple internal systems, including respiratory systems such as gills or trachea
• Often use an open circulatory system to pump their blood
• Found almost everywhere, from the hottest deserts and the deepest seabeds to the darkest caves and the tallest mountains
• Examples include annelids, arthropods, bivalves, coelenterates, echinoderms, squid, sponges, snails, and octopuses

Vertebrates

• Animals with a spine that develops from a notochord they possess as an embryo
• Defined internal systems like complex respiratory structures, a closed circulatory system, and sensory organs that build the nervous system
• Tend to be larger than invertebrates, thanks to their backbone, which allows their bodies to grow larger and move faster than many invertebrates
• Found in all habitats, from forests and deserts to caves and seabed mud
• Examples include mammals, fish, reptiles, amphibians, and birds

In summary, vertebrates are animals with a backbone, while invertebrates are animals without a backbone. Vertebrates tend to be more complex organisms with defined internal systems, while invertebrates have simple internal systems.