60 Causes of Emigrations 



surface to breathe in air. At the mouth of the Amazon River, the 

 lung-fish Lepidosiren and the air-breathing eel Symbranchus live in 

 water that contains very little oxygen and do the same (Carter, 

 1931; Beadle, 1943). Many river snails are branchiate, but most of 

 those that live in ponds and small streams are pulmonates. Lite & 

 Whitney (1925) studied the development of certain rotifers and 

 found that most eggs develop normally if aerated, but that few will 

 hatch unless they are placed in a decomposing mass of organic ma- 

 terial before being aerated, so that the egg shells are broken down 

 somewhat. These animals are thus adapted to conditions which 

 normally obtain in swamps and pools. 



Case (1919, 1926) and Romer (1945) on paleontological evi- 

 dence favor the view that aquatic animals were forced to become 

 terrestrial by progressively increasing aridity. Another possibility is 

 furnished by the observatioAS of Hora (1933) and the writer 

 (1932a, 1933) in India and Siam, where there is heavy rainfall. In 

 the latter country there are twenty-odd species of fishes that have 

 developed various types of lungs for air-breathing as outpocketings 

 from their gill cavities. These live in the shallow klongs, where the 

 writer showed that oxygen may be low or absent altogether at night. 

 Some of the^2 fishes never leave the water but will suffocate if pre- 

 vented from coming to the surface. At least two of these fishes 

 (Ophiocephalus, Anabas) wander freely over the land, especially at 

 night. The "climbing perch" seeks insects and other foods on land. 

 The "serpent heads" live for more than two months without water, 

 buried in dry rice fields. Siamese fishermen fish for them with 

 spades. Perhaps lack of oxygen in limited bodies of fresh water, as 

 well as aridity, has been a factor in causing animals to take up a 

 terrestrial existence. 



A marine sipunculid worm, Physcosoma lurco Sel. & de Man, 

 lives in burrows in the intertidal zone between the roots of man- 

 groves, along the shores of the Sunda Islands. It is actively 

 homoiosmotic and typically marine. Because of lack of oxygen in 



