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 characterized by the absence of a vertebral column, or spine, and they make up a vast majority of animal species on Earth, encompassing various forms of life such as mollusks, arthropods, and annelids.

In contrast, cats, eagles, and platypuses are all vertebrates. Cats and eagles are mammals and birds, respectively, both of which possess a backbone as part of their skeletal structure. The platypus, although a unique mammal with some distinct features, also has a spine. This contrast highlights the defining characteristic of invertebrates— the lack of a spinal column— making the starfish the correct answer in this context.