 The very thick skin (cuticle) is not composed of cells, and is opaque, being secreted by a very thin white epidermis, which is attached by a very thin layer of connective tissue to the white muscle inside.The muscle fibres are attached to the connective tissue and are wound like helixes.In the centre of the muscle runs an inconspicuous tube-shaped opening along the entire length, which is an extension of, and in open connection with, the body cavity within the shell.