Exoskeleton is absent in Branchiostama. Endoskeleton include the notochord, gelatinous rods and plates, fin-ray boxes etc.
1. Notochord in Amphioxus: The notochord of Amphioxus is extending the whole length of the body along the mid-dorsal line just below the central nervous system. The notochord consists of a single longitudinal row of cells. The row of cells is enclosed by a thin, elastic membrane surrounded by a thick layer of fibrous connective tissue called the Notochordal sheath.
Its main function is to give support to the body and maintain its shape.
It permits slight bending movement of the body
2. Gelatinous Rods and Plates: The gelatinous rods give support to the bars of the pharynx, oral cirri and free anterior edge of the oral-hood. The rods and plates are formed by elastic fibers.
3. Fin-ray Boxes: They give support to the dorsal and ventral fins. They contain gelatinous matter and thick connective tissue covering.