In-vertebrates refers to animals without a backbone. The antonyms for in-vertebrates would be "vertebrates" which are animals with a backbone. Vertebrates include humans, mammals, reptiles, birds, amphibians, and fish. Vertebrates are categorized by their skeletal structures, which provide support, protection, and aid in movement. Unlike in-vertebrates, vertebrates have a central nervous system that allows them to perceive their external environment and respond accordingly. Additionally, most vertebrates have a closed circulatory system, which allows for more efficient transport of oxygen, nutrients, and waste products throughout the body. Compared to in-vertebrates, vertebrates tend to have a higher intelligence and complex social structures.