Which of the following organisms is considered an invertebrate?

The starfish is considered an invertebrate because it belongs to the phylum Echinodermata, which includes organisms that lack a backbone. Invertebrates are classified as animals that do not possess a vertebral column, commonly known as a spine. Starfish, or sea stars, exhibit a unique body plan characterized by radial symmetry and a water vascular system, which distinguishes them from vertebrate animals like cats, eagles, and platypuses.

In contrast, cats, eagles, and platypuses are all vertebrates, classified under different groups within the phylum Chordata. Cats and eagles possess a backbone as mammals and birds, respectively, while the platypus is also a mammal but is unique due to its egg-laying ability. The defining feature of vertebrates is the presence of a backbone or spinal column, which is why the starfish stands out as the invertebrate option among the choices provided.