MORPHOLOGY OF ANIMALS 



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FiG. 14. Gill Clefts of Vertebrate Embryos. A, Shark ; B, Chick, C, Man ; g, the gill clefts 

 (black lines) between which are the gill arches (unshaded) ; M, mid-brain, H, hind-brain ; e, 

 eye, er, ear; ft., fore-limb, hi, hind-limb. (After Scott). 



through these pores and by means of the tracheal tubes 

 penetrates to all parts of the body, the exchange of gases 

 taking place directly between the tissues and the tracheae 

 (Fig. 15, B). Among the vertebrates the lungs are an 

 evaginated portion of the pharynx. The swim-bladder, 

 which in most fishes is a hydrostatic apparatus, (Fig. 12, 

 B, /), in the lung fishes (Dipnoi) becomes highly vascular 

 and may serve as a lung. In all higher vertebrates the lung 

 is paired, and its walls which in the lower classes are rela- 

 tively simple, become much folded and very richly sup- 



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