National League for Nursing (NLN PAX) Practice Exam

A vertebrate is an organism that possesses a backbone or spinal column. Among the options presented, the snake is classified as a vertebrate because it is part of the subphylum Vertebrata, which encompasses all animals with a backbone, including mammals, birds, reptiles, amphibians, and fish. The presence of a spine is a defining characteristic of vertebrates, allowing for a more complex structure and a greater range of motion compared to invertebrates.

In contrast, the earthworm is an invertebrate because it lacks a backbone, and its body is instead supported by a hydrostatic skeleton. The starfish, although it has a unique body structure often identified with echinoderms, also does not have a backbone and is therefore classified as an invertebrate. Similarly, the lobster is another invertebrate, belonging to the phylum Arthropoda, which is characterized by their hard exoskeletons and jointed limbs instead of a backbone. This clear distinction explains why the snake is the only option that fits the classification of a vertebrate.