It is a shell, as in a cockle shell or oyster shell.
Exoskeleton/ Chitin
This is called bark.
Schlera
The tough outer covering of bone is called the periosteum. It is a dense connective tissue that protects the bone and plays a role in bone growth and repair.
Periosteum is a membrane that lines the outer surface of all bones,[1] except at the joints of long bones.
The outer covering of euglenas is called a pellicle. It is a flexible layer made up of protein strips called pelliculin. The pellicle helps euglenas maintain their shape and structure while allowing them to move and change shape.
Cell Wall
A bacterial cell is a tiny structure that contains bacterial DNA encased by a tough outer covering called the cell wall. The cell wall provides structure and protection to the bacterial DNA inside.
The tough outer covering that lies just outside the cell membrane in a plant cell is called the cell wall. In plant cells, it is usually made up of cellulose, hemicellulose and pectin.
The fibrous tunic is the outermost layer of the human eye. In specific areas are the sclera, or "white of the eye", and the cornea which is continuous of the sclera.
I think it is the cell wall or the cell membrane
The stratum corneum is the tough outer layer of the skin.