34 Routes from the Sea 



clams, ciliates, etc.; others were found only in the salt marshes. On 

 the coast of California ciliates have been collected in marshes where 

 salinities were 7.5, 12.3, and 20 (Kirby, 1932) . After studying the 

 salt marshes of Croisic, Labbe (1926) concluded that most of the 

 animals present had come from the ocean, and that some were 

 making progress toward fresh water. In Norfolk, England, a 

 littoral anemone (Sagartia luciae Verrill) was found living in water 

 which had a salinity of 14.56 parts per thousand. In Algeria, Beadle 

 (1943) studied 60 limited bodies of water that were more or less 

 saline. He says, "The few marine types (e.g., the diatom Chaeto- 

 ceras sp. and the alga Enteromorpha intestinalis) are found only in 

 relatively low salinities. The majority are of fresh-water origin, 

 including those adapted to the most saline waters." He found layer- 

 ing to be common in pools. A hot saline layer might be overlaid 

 by a cooler, fresher layer. The lower layer might contain high HoS 

 and high O2 simultaneously. 



Probably because swamps and marshes are often deficient in 

 oxygen, many animals which live in them are air breathers. Espe- 

 cially at night in the tropics the oxygen in shallow water often dis- 

 appears altogether. This happens because high temperature makes 

 the solubility of gases in water less and because the activities of 

 bacteria and other organisms use oxygen rapidly. In Siam and 

 India there are about twenty-five species of fishes which breathe 

 air and drown if kept under water for from half an hour to two 

 hours (Das, 1927) . The snails in swamps in both tropical and 

 temperate regions are commonly pulmonates; whereas those that 

 live in streams, where there is plenty of oxygen, are generally 

 branchiate. Probably lack of oxygen more than the periodic drying 

 up of swamps has made paludine animals air breathers (Carter 

 6C Beadle, 1930; Pearse, 1932) . 



Gulick (1948) asserts that all terrestrial gastropods probably 

 came from pond snails. They must remain moist, and they seal 



