244 



CHARACTERS OF VERTEBRATE ANIMALS 



MID BRAIN 



'TWIXT BRAIN 

 PORE BRAIN 



CEREBRAL 

 HEMISPHERE 



BRAIN 



VISCERAL ARCHES 



Two things will now have become apparent if the foregoing" 

 description has been carefully followed: (i) That the aortic arches 

 found in higher Vertebrates, which always breathe by lungs, point 

 to descent from ancestors which were aquatic and breathed by 

 means of gills growing from gill -arches between which there 

 were gill-slits; (2) the Amphibians and Reptiles are, so to speak, 

 still grappling with the problem of adapting a circulatory system 

 originally suited for pumping impure blood to gills, to the new 

 conditions brought about by the adoption of a terrestrial life. 



Even in crocodiles, 

 which by development 

 of two ventricles have 

 succeeded in prevent- 

 ing the two sorts of 

 blood from mixing in 

 the heart itself, have 

 not been successful in 

 preventing them from 

 mixing outside the heart 

 (see p. 208). These 

 facts become clearer 

 still when the develop- 

 ment of the higher Ver- 

 tebrates is studied, and 

 in, say, an embryo chick 

 (fig. 151) gill-arches 

 and clefts are clearly to be seen with their aortic arches, although 

 no true traces of actual gills have been discovered. It may also 

 be pointed out that gill -arches are supported by parts of the 

 internal skeleton in the form of jointed rods which unite with 

 unpaired pieces in the floor of the throat, and the " hyoid 

 apparatus ", which has been described in various animals, is 

 largely made up of bits of this gill-arch skeleton, now turned 

 to further uses. The cartilages which support the larynx or 

 voice-box of terrestrial Vertebrates are also derived from this 

 source, in part at least. These are admirable examples of 

 "change of function " (p. 13). 



The breathing-organs (fig. 1 50) of the adult Salamander have 

 already been spoken of in the foregoing, but it may be- added 

 that the lungs and air-passages agree fairly well with what has 



Fig. 151. — Front part of Chick Embryo 



