ii8 THE WONDER OF LIFE 



Planarians and Nematodes) ; by the earthworms and land- 

 leeches ; by a few Crustaceans, such as wood-lice and land- 

 crabs ; by the archaic Peripatus and its allies — widespread 

 connecting-links between segmented worms and types 

 like Centipedes ; by the Centipedes themselves and their 

 allies, such as Millipedes ; by many Insects ; by Spiders, 

 Scorpions and many Mites ; and by the Pulmonate Gastro- 

 pods, namely land-snails and land-slugs. 



While fishes are, of course, confined to the water, there 

 are some interesting curiosities. Thus the eel may make 

 short excursions over the moist grass of the meadow, 

 and some tropical fishes burrow deep into the mud in 

 the dry season. In the common Periophthabnus of 

 tropical shores we have one of those extraordinary excep- 

 tional cases — a fish that can remain for many hours out 

 of water. The same is true of the interesting double- 

 breathing mud-fishes (Dipnoi), which have their swim- 

 bladder turned into a sort of lung, and can five long out 

 of water. Among backboned animals, the transition from 

 aquatic to terrestrial life was made in the Carboniferous 

 Period by the Amphibians, many of which stiU recapitulate 

 every year the historically important step — passing from 

 a larval or tadpole gill-breathing life in the water to an 

 adult lung-breathing life on land. In a few cases, e.g. 

 the black salamander [Salamandra atra) of the Alps, which 

 lives above the level of water-pools, and some tree-frogs 

 which never come to earth, the aquatic gill-breathing stage 

 is skipped altogether. 



In Eeptiles, Birds, and Mammals, as every one knows, 

 there is no trace of gills left in early life (though the tell- 

 tale gill-clefts remain in the embryo), and the yoimg are 

 lung-breathers from the time they are born or hatched. A 



