This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
All of the following ocular structures are derived embryologically from the neuroectoderm except: 
Select and explain. 
A. Epithelial layers of ciliary body and iris, B. Sphincter and dilator pupillae muscles, C. Optic nerve, D. Optic nerve sheaths
D: Optic nerve sheaths
Answer: D: Optic nerve sheaths 
Explanation: Ans. Optic nerve sheaths