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the eight species of phanerogams, which have acquired a 

 wholly marine habit, and are now known as sea-grasses. In 

 the case of these plants, the adaptation is so pronounced 

 that thev cannot live out of their adopted habitat. The 

 mangrove plants, on the other hand, are only partially 

 accommodated to the conditions of the marine littoral dis- 

 trict, for it is necessary that their crowns of leaves should be 

 above water. 



Marine animals and plants likewise attempt migrating 

 from the sea to the land. In their adaptatiou to the new 

 habitat, the animals are the most successful, just as the 

 plants are the most successful in migrating the other way. 

 Thus two genera of fish, Periophthalmus and Boleophthal- 

 mus, are able to pass the greater part of their lives out of 

 water. They ''skip along close to the water-line on the 

 sea shore, where they hunt for molluscs (Onchidiuui) and 

 insects.'' (Semper, "81, p. 189.) The large branchial cavity 

 of these fishes is not completely filled by the gills, but serves 

 in part as an air cavity or primitiA'e lung. In certain 

 fishes, such nt^ uinabas scandens of the Philippines, this gill- 

 cavity is further modified into a "labyrinthine organ," or 

 much prolonged cavity, the mucous membrane of which is 

 thrown into complicated folds, thus greatly increasing the 

 surface. These fish are able to exist for days out of water, 

 and can 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., Lit- 

 torina, llyanassa, etc. .are capable of existing out of water for 

 a considerable time, commonh' crawling out of the vessels 

 in which they are kept. 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 laid 

 bare at low tide. Ampullaria forms a connecting link between 

 marine and land snails, for it not onlv breathes bv means of 



