Growing cylindrical shaft slides out of the tube-like follicle. Follicle extends deep into the hypodermis. Follicle associated with sensory nerves. Muscles associated with the follicle provide movement and orientation of the shaft. Shafts vary with the stage of development showing, for example, different structure and colouration in the neonate, juvenile and adult.
Shafts shed or mould seasonally resulting in changes in structure and colour.
Sexual dimorphism of shafts under endocrine control. Growth cycle with a growing and resting stage. The growth cycle can be initiated at any time by plucking the shaft from the follicle. In the growth cycle, the old shaft is ejected by the growth of a new one. Exocrine function of follicle and shaft.