986 PRINCIPLES OF STRATIGRAPHY 



IMarine animals and plants, likewise, attempt migrating from the 

 sea to the land. In their adaptation to the new habitat the animals 

 are the more successful, just as the plants are the more successful 

 in migrating the other way. Thus two genera of fish, Periophthal- 

 mus and Boleophthalmus, are able to pass the greater part of their 

 lives out of water. They "skip along close to water-line on the 

 seashore, where they hunt for molluscs (Onchidium) and insects." 

 {Semper-/ : I Sg.) The large branchial cavity of these fishes is not 

 completely filled by the gills, but serves in part as an air cavity or 

 primitive lung. In a number of fishes, such as Anabas scandens of 

 the Philippines, this gill-cavity is further modified into a "laby- 

 rinthine organ," or much prolonged cavity, the mucous membrane 

 of which is thrown into complicated folds, thus greatly increasing 

 the surface. These fish can exist for days out of water and are 

 able to make long overland excursions. Semper holds that these 

 fish may be regarded "as Amphibians with quite as much reason as 

 toads and frogs, or even better, since they are capable of changing 

 the nature of their respiration — of air, that is, or of water — at will, 

 and suddenly without any interruption." Several of our littoral 

 gastropods, e. g., Littorina, Ilyanassa, etc., are capable of existing 

 out of water for a considerable time. In Brazil Littorina climbs 

 the trees of the mangrove high above water and oysters and other 

 bivalves are attached to the roots of these trees and are laid bare 

 at low tide. Ampullaria forms a connecting link between sea 

 and land snails, for it not only breathes by means of a gill but also 

 has a pulmonary sac like that of the land snails into which air is 

 carried by means of a long breathing-siphon. 



The possibility that related species of marine gastropod Afollusca 

 may leave the sea in different parts of the world and give rise to 

 terrestrial forms, which, though differing from their marine an- 

 cestors, may be very similar to each other, deserves attention. 

 In this way some of the puzzling problems of distribution of ter- 

 restrial gastropods on widely distant oceanic islands may be ac- 

 counted for. 



Among the Crustacea there are several species of crabs {e. g., 

 Birgns latro, etc.) which live in damp woods far from all water 

 and whose respiration is carried on chiefly without the intermedia- 

 tion of their normal medium. 



The advent of marine vegetation on the land has occurred only 

 up to the limit of the salt spray on exposed shores, and here the 

 number of species is small. But at, or just below, high-tide limit 

 a number of alg?e find a congenial abode, and grow there in luxuri- 

 ant masses. Chief of these in our northern latitudes are the Fuci, 



