Each duct consists of four parts:
(a) an outer ductule, thin-walled, barely sclerotised, round in cross section and generally at least 10 μm long, which opens through the dorsum by a small inconspicuous pore that may occasionally be mildly sclerotised; at its inner end is (b) a characteristic structure, here referred to as the cup shaped invagination because the outer ductule terminates in a thick-walled structure that is bowl- or cup-shaped, and usually slightly asymmetrical; from one side of the cup arises (c) the inner ductule which is usually narrower and shorter than the outer ductule; this terminates in (d) a `flower-head'-like structure, here referred to as the terminal gland. Tubular ducts can vary in the relative lengths and widths of the inner and outer ductules, in the form (particularly the depth) of the cup-shaped invagination, and in the size of the terminal gland.
For photographs, see ventral tubular ducts