Question

question 6
which characteristic is unique to vertebrates?
they have an exoskeleton.
they have a well - defined skeleton.
they have no defined organ systems.
they have bilateral symmetry.

Explanation:

1. Analyze "They have an exoskeleton": Organisms like arthropods (e.g., insects) have exoskeletons, while vertebrates have endoskeletons. So this is not unique to vertebrates.
2. Analyze "They have a well - defined skeleton": Vertebrates have an internal (endoskeleton) that is well - defined, made of bone or cartilage. Many invertebrates either have no skeleton or a less - defined one (like the hydrostatic skeleton in some worms). This is a key unique feature of vertebrates.
3. Analyze "They have no defined organ systems": Vertebrates have highly defined organ systems (e.g., circulatory, respiratory, nervous systems). So this is incorrect.
4. Analyze "They have bilateral symmetry": Many invertebrates (e.g., annelids, arthropods) also have bilateral symmetry. So this is not unique to vertebrates.

Answer:

B. They have a well - defined skeleton.