Head and mantle
Red triangle slugs have a mouth with thousands of tiny teeth called radula. They use their radula to scrape algae off smooth surfaces.
They have two optical tentacles are retractable with light sensitive eye spots. They are also used for smelling and feeling the surrounding environment.
The respiratory pore, or pneumostome, is used for breathing. It is located in the red triangle mark on their back. This is part of the slugs mantle.
The mantle is made of a thicker flesh than the head. When frightened or not active the slug retracts its head toward the mantle for protection.