LITTORAL FAUKA. 125 



better fitted to their new surroundings. This we see in the defen- 

 sive armour or encasement with which a very large number of the 

 shore animals are provided, a character which eminently distin- 

 guishes them from the inhabitants of the open ocean. The shells, 

 which are with the latter in most cases very thin and fragile, as in 

 Atlanta, the argonaut, and the pteropods generally, are in the vast 

 majority of shore animals thick and resisting, and capable of with- 

 standing the numerous strains and impacts to which they are sub- 

 jected. 



But the same causes which have been operative in producing 

 modifications in the pelagic fauna have been influential in bringing 

 about a no less important series of modifications in the littoral 

 fauna as well. The intermingling of fresh and salt waters about 

 the embouchures of rivers, or a deficiency, as in the ice-bound north, 

 in the salinity of the sea itself, will have gradually paved the way 

 for the formation or evolution of animal forms destined to live 

 eventually in fresh water. Hence, the origin in principal part of 

 the fresh-water faunas. Similarly, frequent exposure to the at- 

 mosphere beyond the interacting influence of the aqueous medium, 

 as in the region of "between tides," will have developed a method 

 of respiration, or respiratory apparatus, other than that which is 

 dependent for its action upon the presence of water. The remark- 

 able series of modifications which the Amphibia (frogs, toads, sala- 

 manders) undergo from their larval condition, when, as inhabitants 

 of the water, they breathe by gills, to their adult stage, when 

 respiration is in most cases effected through the intermedium of 

 lungs alone, most forcibly illustrate the progression which, at least 

 in one division of the animal series, the vertebrates, has led to 

 the formation of the air-breathing or terrestrial fauna. But other 

 instances of adaptation from an aqueous to a terrestrial existence 

 are not wanting. Many fishes have their gills so modified as 

 to permit of a very protracted existence on dry land, while, as is 

 well known, the lung-fishes (Dipnoi) have developed true lungs. 

 Land-crabs are very abundant in the Tropics, roaming about in the 

 interior at very considerable distances from the shore. In Japan 

 they have been observed at an elevation of four thousand feet above 

 the sea. The remarkable cocoanut crab, Birgus latro, is provided 

 with a pair of true lungs, developed on the walls of its gill cavities. 



Professor Moseley 48 thus sums up the relations of the littoral 



