Is a turtle considered a shellfish?

Why are scientists concerned about the depletion of the ozone levels in the stratosphere?

Explain how paleontologists use trilobite fossils as index fossils for various geologic time periods.

Ivan is looking at a cross section of a stem taken from a vascular plant. he sees two different types of vascular tissue: the xylem, which is closer to the center of the stem, and the phloem, located around the xylem, closer to the outside of the stem. how do these two structures work together in a living plant?