86 Causes of Emigrations 



regard to their water requirements. Eight species of snails which 

 Colgan (1910) studied on the Irish coast were arranged in rather 

 definite zones between low- and high-tide marks and showed in 

 the same order progressive ability to survive in dry air; their limits 

 ranging from 6 to 42 days. A mud-flat snail (Ilyanassa) from 

 the coast of the United States survived only five days when treated 

 similarly (Dimon, 1905). Two species of beach hoppers (Tal- 

 orchestia) studied by Verwey (1927) were found to occupy dif- 

 ferent zones on the beach and to require different amounts of water 

 in the sand in which they lived. Some marine, littoral animals 

 drown in water but must visit the ocean frequently in order to 

 moisten their bodies or their respiratory surfaces. Among these, 

 beach-skipping gobies, fiddler crabs (Uca) , and the ghost crabs 

 (Ocypode) may be mentioned. A barnacle, kept out of water for 

 a week or two, goes into a resting state and then will remain alive 

 for some time. Monterosso (1927, 1929) thus kept barnacles 

 alive for from 100 to 140 days. When he submerged them for a 

 short time at intervals of 30 to 90 days, some individuals remained 

 alive for more than two years. Some barnacles which are attached 

 near high-tide mark are usually submerged for only one hour out 

 of twenty (Flattely & Walton, 1922). The little isopod Ligia 

 exotica, which runs about near the water on tropical beaches, may 

 live 8 hours to 4 days out of water, depending on temperature and 

 amount of moisture in the atmosphere. In a saturated atmosphere 

 death occurs when 8% of the body weight has been lost. The body 

 contains about 75% of water, and a loss of 10 to 18% is always 

 fatal. This crustacean must live near the sea because, though 

 terrestrial, it has no special water-retaining adaptations. In Java, 

 Harms (1929) found that beach gobies lived only 1 to 5 days on 

 damp mud without water. Three species of blennies live under 

 stones about a meter above and below low-tide mark on the beaches 

 in Puget Sound, Miles (1920) believes because oxygen is more 

 abundant there than in the ocean below. 



