PHYLUM CHORDATA 



317 



OSTEiCHTHYES (sunfish) 



Primitive 

 bony fishes 



• .• •....•.■.•.ei)" 



MAMMALIA (squirrel) 



AMPHIBIA (frog) 



CHONDRICHTHYES (shark) 



PLACODERMI 

 (primitive-jawed fishes) 



AGNATHA 

 (jawless vertebrates) 



Hypothetical Chordate Ancestor 



Figure 200. A simplified family tree of the vertebrates. The ancestor to the most primitive 

 jawless vertebrates (class Agnatha), which are fossil ostracoderms, is unknown. The primitive 

 jawed placoderm is also a fossil form. (The ostracoderm and placoderm after A.S. Romer.) 



connective tissue. The myotomes on one 

 side of the body alternate with those on the 

 other side. The muscle fibers contained in 

 them are longitudinal; and since they are 

 attached to the connective tissue partitions, 

 they are able to produce the lateral move- 

 ments of the body used in swimming. 



The mouth proper is an opening in a 

 membrane posterior in the oral hood. The 

 anus is situated on the left side of the bodv 

 near the base of the caudal fin. The 

 atriopore is just anterior to the ventral fin; 

 it is a ventral opening through which water 

 used in respiration passes to the outside. 



