152 THE STORY OF ANIMAL LIFE 



the centra. When the centrum, instead of being 

 biconcave, is solid, as in the higher Vertebrata, 

 the notochord is obliterated by its encroachment. 



The Amphibia, familiarly represented by Frogs 

 and Toads, receive their name, " adapted for both 

 lives," from the fact that they usually divide their 

 lives between land and water. They are, from 

 one point of view, the most interesting of the 

 classes of the Vertebrata, for they form a divid- 

 ing line between the lower and upper Chordata. 

 Below we have Hemichordata, Ascidians, Amphi- 

 oxus, Fishes ; all water-dwellers, breathing by 

 gilKs. Above, we have Reptiles, Birds, Mammals, 

 air-breathers, never possessing gills, except for 

 a short time, as rudiments in the embryo, not 

 brought into use. They are linked by the Am- 

 phibia, in which we see the larva a water-dweller, 

 breathing by gills; the adult, an air-breather, 

 adapted for life on land, and obliged to come to 

 the surface to breathe, even when it passes its 

 time in the water. The individual Amphibian 

 tells us the past history of the higher groups ; 

 once they had gills but growing older, they lost 

 them. 



Fig. 43 shows us an outline sketch of Am- 

 phibian larvae; we should require an enlarged 

 diagram of an earlier stage, to show the gills, 

 which are external and projecting at first, but 

 afterwards are overgrown by the skin with the 

 exception of an orifice on each side. The dia- 

 gram shows the gradual change of form. The 

 tails in these tadpoles will presently be lost, 

 for they belong to the Anura, or tail-less order 

 of Amphibia (Frogs and Toads). The tailed 

 Amphibians, Urodela, are represented in Great 

 Britain by the Newts, Triton, popularly called 



