38 Routes from the Sea 



Subterranean Habitats 



Subterranean animals live where environmental conditions are 

 quite stable. They are in little danger of injury by desiccation; they 

 are not subject to sudden and extreme changes in tem.perature; and 

 they are by their mode of life concealed from many predaceous 

 enemies. At times soil animals suffer from lack of oxygen, especially 

 during rainy weather when the water content of the soil is high; 

 sometimes soil reactions change beyond their limits of toleration; 

 but in general they enjoy a considerable degree of stability and 

 safety. They of course have to pay the penalty that nature exacts 

 from all specialists. Their distance-perceptive organs, sight and 

 hearing, are often feeble, and they depend largely on contact senses, 

 such as touch, taste, and smell, in their responses to environment. 

 The soil has been invaded by arthropods and vertebrates which 

 come from ancestral stocks which long ago enjoyed an epigean 

 existence. On the other hand, some soil animals such as earthworms, 

 apterygotid insects, tipulid and tabanid fly larvae perhaps spread 

 from aquatic habitats and thus became dwellers in soil instead of in 

 muddy, sandy, or rocky bottoms. 



But even along the seashore burrowers are specialists. Certain 

 sponges, worms, echinoderms, mollusks, and crustaceans are able to 

 bore into rock (Russell 6C Yonge, 1928) , and thus find safe retreats 

 on wave-swept shores. Marine burrowers generally attain environ- 

 mental stability. Reid (1930) found that the water from a fresh- 

 water stream which flows over a sandy beach on the seashore pro- 

 duced no efl^ect on the salinity in the sand below 25 cm. Some 

 worms at low tide burrowed deeper, thus avoided the diluted water, 

 and were able to establish themselves in the estuary of a stream. 

 But fossorial habits always tend to make animals specialists. Marine 

 worms, crustaceans, and clams often have elaborate adaptations 

 for protecting their respiratory cavities (Garstang, 1905) and for 



