A tube formed by the inward extension of the cell membrane is called a vesicle.

Here's why:

* Vesicles are small, membrane-bound sacs that transport substances within the cell or between cells. They are formed by the budding off of existing membranes, like the cell membrane. This process of inward extension and pinching off is called endocytosis.

* Other terms you might encounter related to this process:

* Endosomes: Larger vesicles that form during endocytosis.

* Phagocytosis: A type of endocytosis where the cell engulfs large particles, like bacteria.

* Pinocytosis: A type of endocytosis where the cell takes in small particles or liquids.

Let me know if you have any other questions about cell structures or processes!