Which animal is considered an invertebrate?

Starfish is considered an invertebrate because it belongs to the phylum Echinodermata, which includes animals that do not possess a backbone. Invertebrates are defined as animals lacking a vertebral column, or spine. Starfish, specifically, have a unique radial symmetry and a tough, calcified body that distinguishes them from vertebrates, which typically possess a more rigid skeletal structure.

The other animals listed, such as frog, elephant, and turtle, are all vertebrates. Vertebrates are characterized by having an internal skeleton made of bone or cartilage, along with a backbone. This fundamental difference in body structure is what categorizes starfish as an invertebrate. Understanding these classifications is important in biology as it reflects both evolutionary relationships and biological functions within ecosystems.