Final answer:
The core of a cilium or flagellum is called the axoneme, which is made up of microtubules. The axoneme has a '9 + 2 array' structure, with a ring of nine microtubule doublets surrounding a pair of singlet microtubules in the center.
Step-by-step explanation:
The core of a cilium or flagellum is called the axoneme. The axoneme is made up of microtubules and has a structural arrangement known as a '9 + 2 array'. It consists of a ring of nine microtubule doublets surrounding a pair of singlet microtubules in the center. This arrangement allows flagella and cilia to move and propel cells.