RESPIRATION IN SHORE ANIMALS 207 



refresh themselves with a gulp of air. As a matter of fact, 

 if the two fishes Boleophthalmus and Periophthalmus are 

 prevented from reaching the surface in order to take in air 

 they are drowned (Willey, 191 1), a curious end for such 

 typically aquatic forms as fish. 



In more or less similar fashion, the different common 

 species of Littorina when placed in an aquarium will 

 voluntarily climb up the sides to some distance above the 

 water-level. They also die if permanently cut off from 

 access to air. Many other amphibious snails occur outside 



Fig. 14. — Periophthalmus, a tropical fish which moves about over 

 mud flats at low tide by the aid of its pectoral fins. Note the modified 

 pelvic fins acting as a sucker. 



our area. One of the most remarkable of these is Ampul- 

 laria which has gill and lung cavities equally well developed 

 and when living in water makes use of both in rapid alterna- 

 tion (see Semper, op. cit.). 



Certain species of crabs exhibit an almost exact analogy 

 with Amphibians proper, spending most of their time on 

 land, and only returning to the water for very short intervals 

 or perhaps only at breeding time. They owe their partial 

 conquest of the land largely to the protected situation of 

 their respiratory organs which prevents their becoming 

 dried up, and partly also to the gradual accommodation to 

 sub-aerial conditions which shore life affords. The begin- 

 nings of air-breathing are already observable in the common 

 crabs of our own sea-shore. But in order to see how far 

 the process of emancipation from water-breathing may 



