CHAPTER XXI. 

 PHYLUM XII: CHORD ATA (VERTEBRATES). 



288. General Statement. To this point we have been 

 studying the invertebrates, animals that have their hard 

 parts on the outside. In the remaining phylum the 

 skeleton is chiefly internal. It includes the fishes, the 

 frogs, the lizards, the turtles, the birds, the mammals. In 

 some of these there is an external skeleton also. The 

 internal skeleton is more simple in the lower members of 

 the branch. It consists typically of a long rod of tissue 

 dorsal to the digestive tract and ventral to the nerve cord. 

 In its simplest form it is known as a notochord and is of 

 very delicate tissue. This gives the name Chordata to the 

 whole group. In most the notochord becomes surrounded 

 with cartilage or bone, and forms a real spinal (vertebral) 

 column. These are the real vertebrates. 



In the animals studied hitherto the nervous system has 

 been on the ventral side of the animal and the heart 

 dorsal. In the vertebrates the chief nervous centres are 

 dorsal, and the heart is ventral to the digestive tract. 

 (Compare Figs. 106 and 6.) In the vertebrates the 

 breathing structures, whether lungs or gills, rise from the 

 anterior end of the digestive tract (the pharynx}; and in 

 all the vertebrates, at some stage of life, there are slits 

 from the pharynx to the outside world. These slits bear 

 gills in the lower groups, but close up and disappear in the 

 reptiles, birds, and mammals. 

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