A single layer of epidermic cells, some of which are glandular, forms the outer layer.
Beneath the epidermis is a longitudinal layer of muscle-fibres which are separated into four distinct groups by the dorsal, ventral and lateral areas; these are occupied by a continuation of the epidermic layer; in the lateral areas run two thin-walled tubes with clear contents, which unite in the anterior part of the body and open by a pore situated on the ventral surface usually about a quarter or a third of the body length from the anterior end.
The setae ar ° invariably formed each within an epidermic cell, and they are sheathed in involutions of the epidermis.
They are found one on each cephalic tentacle, and are simply minute open pits or depressions of the epidermis, the epidermic cells lining them being pigmented and connected with nerves (compare fig.
Salicylic acid is used externally for the removal of corns and similar epidermic thickenings.