Animals whose basic body plan can be described as a tube within a tube are called bilaterians.

Here's why:

* Tube within a tube: Bilaterians have a digestive tract that runs from mouth to anus, forming a tube. This inner tube is surrounded by another tube, which is the body wall. This arrangement allows for specialized functions within the body.

* Bilateral symmetry: Bilaterians exhibit bilateral symmetry, meaning their bodies can be divided into two mirror-image halves. This symmetry is closely linked to their tube-within-a-tube body plan.

* Examples: This body plan is very common and includes most animals, such as humans, worms, insects, and mollusks.

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