The correct answer is "thoracic duct".
The thoracic duct (also called as the left lymphatic duct or the alimentary duct) is the structure wherein most of the lymph in the drained from the lymphatic vessels goes to this structure. In the thoracic duct, lymph flows up to the level of the brachiocephalic vein where lymph returns to the venous circulation.
By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory. The unified cell theory states that: all living things are composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing cells.
Answer:
Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed for a brief time period. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
