Subterranean Habitats 39 



breathing in water. Many make characteristic protective tubes of 

 various types. 



A few crabs and other crustaceans (Uca, Macrophthalmus, 

 Gecarcinus, Ocypode, Thalassina, Birgus, etc.; Pearse, 1914a, 1929, 

 1929a, 1931; Verwey, 1927) have quite evidently spread from bur- 

 rows along the seashore to burrows farther inland, but none of 

 these have become dominant land animals which play an important 

 role in terrestrial Ufe as a whole. 



The earthworms are probably about the only group of dominant 

 land animals that may have spread from aquatic habitats through 

 the soil. Such careful studies of soil animals as Cameron (1913), 

 Bornebusch (1930), and others have made show that, except for 

 earthworms, dominant soil animals belong largely to modern groups 

 such as Diptera, Coleoptera, and certain Myriapoda. Primitive 

 apterous insects are present in great numbers near the surface and 

 extend down to depths of as much as two meters (Shelf ord, 1929) , 

 but they constitute a very small part of the mass of animals that 

 occurs in soils. It is possible, however, that such minute insects have 

 spread from ancient seashores, as some live in such situations today, 

 and thus have gradually invaded the soils far from water. 



A few animals have become established as specialized permanent 

 residents of caves. Certain of the more specialized of these lack 

 pigment, and have degenerate visual organs and enlarged tactual 

 organs. Some of them also occur in underground waters and at 

 times are taken from wells or appear in epigean habitats. Some 

 cave fishes and crustaceans may be said to have rather definite af- 

 finities with marine animals (Eigenmann, 1898, 1909; Pearse et al., 

 1935) and doubtless are relics of a fauna which inhabited the caves 

 when they were submarine or littoral (Davis, 1931). However, the 

 fauna of caves as a whole consists of representatives of rather 

 recent and specialized groups (bats, salamanders, beetles, flies, 

 spiders, and crayfishes) , as well as archaic, primitive types. "Tern- 



