Which of the following is NOT a type of exoskeleton?

Step 1: Analyze the question We need the one option that is NOT an exoskeleton. An exoskeleton is a hard, supporting or protective covering on the outside of the body.

Step 2: Explain the concept

• Exoskeleton: external covering, such as the chitinous cuticle of insects, the shells of snails and crabs, and the hard keratin structures of vertebrates (hooves, nails, claws, horns, scales).
• Endoskeleton: internal support, made of bone and cartilage, found inside the body of vertebrates.

Step 3: Check each option

• A. Cartilage: This is a flexible connective tissue found inside the body (ear, nose, joints, and the whole skeleton of sharks). It is part of the endoskeleton, so it is NOT an exoskeleton.
• B. Chitin: This forms the exoskeleton of arthropods such as insects and crustaceans. It is an exoskeleton.
• C. Hoot: This is not a skeletal term. It is probably a misprint of "hoof", which would be an exoskeletal structure made of keratin. As written, "hoot" is a sound an owl makes, so it is not an exoskeleton either.
• D. Nail: This is a keratin covering on the outside of fingers and toes, so it is exoskeletal.
• E. Shell: This is the hard external covering of snails, tortoises and others, so it is an exoskeleton.

Step 4: Identify the answer "Hoot" looks like a typing error for "hoof", so it is probably not what the examiner meant. Reading the question as intended, the only option that is a true internal skeletal tissue and not an exoskeleton is Cartilage.

Answer: A. Cartilage

Exam tip: Ask yourself "Is it inside or outside the body?" Cartilage and bone are inside (endoskeleton). Chitin, shell, nail, hoof, horn and scales are outside (exoskeleton).