in the membranous skeleton sheath surrounds the central nervous system I perineural ii perineurium iii peritoneum iv perichondrium

The answer is perineural since peri means around and neural CNS

the trilobite is used as an index fossil for various geologic time periods as specific types of the trilobites were present in different geological time periods. they are a group of marine arthropods belonging to the class of trilobita.

explanation:

there’s no image attached but i can if you want: )