CLIMAX OF THE ANIMAL KINGDOM— PHYLUM CHORDATA 



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the dorsal aorta to supply the parts of the body posterior to the heart. 

 In the ventral aorta it is pumped forward. This is just the reverse of 

 the directions of blood flow in the higher invertebrates. In chordates 

 the blood remains at all times within vessels, and therefore they have 

 a closed type of system like the Annelida rather than an open type of 

 system such as is found in the Arthropoda. 



An Endoskeleton. The skeleton of the chordates, when present, is 

 internal to some of the muscles and other body structures and, therefore, 

 is called an endoskeleton. The advantages and disadvantages of this 



Fig. 21.3. Diagrammatic comparison of a higher invertebrate with a lower vertebrate 

 animal. Note, in the invertebrate, the nerve cord is ventral and the blood flows ante- 

 riorly in the dorsal vessel. In the vertebrate the nerve cord is dorsal, the blood flows 

 posteriorly in the dorsal vessel, there is a notocord, there are gill clefts in the pharyn- 

 geal region, and there is a tail beyond the anus. 



type of skeleton have been discussed in Chapter 13 in comparison with 

 the exoskeleton of some of the higher invertebrates. Some vertebrates 

 have developed a type of exoskeleton in addition to their endoskeleton. 

 The turtle is quite well protected by an enlargement and fusion of scales 

 to form its "shell." 



Subphyla of Chordates 



The phylum Chordata is divided into four subphyla. The first three 

 of these are small and little known and are ordinarilv not considered in 



