An example of an exoskeletal structure is

Analyze the question: The question asks for an example of an exoskeletal structure. "Exo-" means outside, so an exoskeleton is a hard supporting or protective structure found on the outside of the body.

Explain the concept:

• Exoskeleton: an external skeleton that covers and protects the body. Examples are the cuticle of insects (made of chitin), the shells of snails, crabs, and tortoises, and the scales of fish.
• Endoskeleton: an internal skeleton made of bone or cartilage inside the body, as in mammals, birds, reptiles, amphibians, and fish.

Solution: check each option

• A. Femur: the thigh bone, part of the internal skeleton (endoskeleton).
• B. Scapula: the shoulder blade, part of the pectoral girdle and so endoskeletal.
• C. Shell: a hard covering on the outside of the body, as in snails or tortoises. This is exoskeletal.
• D. Skull: the bony case of the head, which is internal and endoskeletal.
• E. Tibia: the shin bone of the lower leg, also endoskeletal.

Only the shell lies on the outside of the body, so it is the exoskeletal structure.

Tip: Split the options into bones and non-bones. Femur, scapula, skull, and tibia are all bones of the vertebrate internal skeleton, and the one that stands out is the exoskeleton. Remember "exo = exit/outside" and "endo = inside."