No tracheate Crustacea are known, but some terrestrial Isopoda develop pulmonary in-sinkings of the integument.
The immediate cavities or pits into which the tracheal stigmata open appear to be in many cases ectodermic in sinkings, but there seems to be no reason (based on embryological observation) for regarding the tracheae as an ingrowth of the ectoderm.
These interior landslips, besides choking the pipes and breaking the communication, often produce sinkings at the surface.
Jebel Akhdar, being without "faults," has no deep internal valleys, and presents, the appearance of downs: but its seaward face is very deeply eroded, and deep circular sinkings (swallow-holes) are common.
Some of the earlier sinkings of this kind, when pumps had to be depended on for keeping down the water, were conducted at great cost, as, for instance, at South Hetton, and more recently Ryhope, near Sunderland, through the magnesian limestone of Durham.