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ESSENTIALS OF ZOOLOGY 



tive set of six through the fishes, Amphibia, reptiles, to the birds 

 and mammals, with a modified condition of three arches. 



Along with the aortic arch situation are other examples of similar 

 stages of development in all forms of the group. Gills are present 

 in all chordates at some time. In the primitive ones the gills are 

 functional throughout life; in the more advanced types they are 



Fig. 215. — Diagram to show the modifications of aortic or branchial arches in 

 different vertebrate types. A, primitive scheme ; B, lungflsh ; C, primitive amphibian 

 (urodele) ; D, frog; E, snake; F, lizard; G, bird; H, mammal, ec, ic, external and 

 internal carotids ; va, ventral aorta ; da, dorsal aorta ; db, ductus Botalli ; p, pul- 

 monary artery; s, subclavian; c, coeliac. Vessels carrying venous blood are black; 

 those with mixed blood are shaded ; those which disappear are dotted outlines. 

 (After Boas, Reprinted by permission from Kingsly, Comparative Anatomy of 

 Vertebrates, P. Blakiston's Son and Company.) 



only transitory gill structures. In frogs and toads, the tadpole stage 

 is essentially fishlike, and they become amphibian at metamorphosis. 

 The notochord is a definite structure throughout the life of the 



