Verified answer

The answer is B, but it is not very astronomical (where you had it the first time)

The order, going from outside to inside is

ecto-, meso-, endo-

these Greek roots mean "outside", "middle" and "inside" (or under).**

Most of the time, the mesoderm is actually a muscle, stuck to the outer skin (ectoderm), while the coelom is the cavity between the muscle and the inner skin that surrounds the organ (for example, the gut).

The role of the coelum is to act as a shock absorber (protecting the inner "thing" from shocks coming from the outside world) and as a non-bone structural support (like an inflated wall supporting the shape of the gut).

Since the coelum provides pressure pushing the outer skin outwards, then the mesoderm (the muscle) only needs to contract (pushing inwards). When the muscle contracts, pressure pushes in. When the muscle stops contracting, the coelum pushes it back out. In this way, there is no need for a double-acting muscle (which are difficult to evolve). It keeps things much simpler.

---

ecto- meso- endo

An "ectoparasite" is a parasite that stays on the outside of its host (for example, a flea or lice).

"Mesopotamia" was called that (by historians) because it was a land located "between" two rivers (the Tigris and the Euphrates).

An "endodontist" is a dentist that looks after the inside of the teeth (the pulp of the tooth) and is the specialist that often does "root canal jobs" which involves going inside the tooth.