264 Elementary Biology. 



higher in the scale than Amphioxus this notochord becomes 

 much modified. The originally cylindrical rod is encroached 

 upon by the fibrous sheath which covers it, and the sheath 

 itself becomes transformed into or replaced by cartilage, or 

 'gristle' (in the lower fishes, e.g. skate and shark), and 

 by bone (in bony fishes, e.g. cod, and all the higher 

 animals). Moreover the flexibility of the primitive axial 

 rod, which would otherwise have been lost in the firm 

 osseous cylinder, is still obtained by the segmentation of 

 the osseous column into separate short cylinders, or ver- 

 tebrae, capable of a limited amount of movement on each 

 other. 



In accordance with the plan adopted in the preceding 

 sections, we may begin our study of one of the higher 

 animal types by a brief summary of the external characters. 

 In regard to the choice of a suitable type for the discussion 

 of the structure and general physiology of the higher Chor- 

 data there is much to be said in favour of the newt or the 

 salamander. The Amphibia, to which these examples 

 belong, is a remarkably interesting group in many respects. 

 Among them, for example, we find the first appearance of 

 true lungs, the characteristic respiratory organs of the 

 reptiles, birds, and mammalia, and the life-history of many 

 members of the class presents us with stages where the 

 transition from the fish-like condition respiration by gills, 

 or branchiae to the true air-breathing condition can be 

 easily demonstrated. Again, the skeleton in the Amphibia, 

 and especially in those members of the group cited above, 

 is found in a very typical condition, and the modifications 

 which have taken place in birds and mammals can be more 

 easily understood after a careful study of the system in its 

 relatively simple state. 



The difficulty of obtaining specimens of the newt or 

 salamander is a serious drawback, and we are accordingly 

 compelled to fall back on the common frog (Rana tempo- 

 raria) for dissecting purposes, although the skeleton of that 



