THE ANIMAL CLIMAX-THE CHORDATES 



287 



Fig. 13-9. Sea lampreys {Pefromyzon marinus), attacking a fresh-water fish. Note the scar from a previous injury 



on the dorsal side just above the fore-fins. 



developed eyes, and a pair of simple semi- 

 circular canals on each side of die head, 

 used in balancing. It has five pairs of gills 

 — less than amphioxus, but more than the 

 common bony fish. Internally, also, the 

 complexity of its body structures far ex- 

 ceeds that of amphioxus in all respects. 



Studying fossil records in an effort to 

 determine whether any cyclostome-like 

 forms occurred in the past, paleontologists 

 have been rather successful. A group of 

 animals called ostracoderms (Fig. 13-11) 

 that lived about 400 million years ago (Si- 

 lurian Period), resembled the present-day in these animals today. This appears to be 

 cyclostomes in many respects. They devel- true, since the early fossil remains present 



only hindered rapid progress, and perhaps 

 this was a factor in its disappearance. At 

 one time, the cartilage of the lamprey skele- 

 ton, as well as that of the sharks and skates, 

 which came later, was considered a precur- 

 sor to bone and therefore a more primitive 

 condition. More recently this has been in- 

 terpreted as a degenerate condition. It is 

 now thought that the cvclostomes and 

 sharks probably descended from forms that 

 possessed not only internal skeletons of 

 bone but also heavy outside bony plates, 

 which degenerated into the cartilage found 



oped heavy armor plates on their external 

 surfaces, possessed a ventral mouth, and 

 were without appendages. Their heavy exo- 

 skeletons were essential to survive the on- 

 slaughts of their invertebrate enemies, the 

 water scorpions ( eurypterids ) . It is pretty 

 well agreed now that later descendants of 

 the ostracoderms lost their plates as these do very little free swimming in the water 

 enemies disappeared. With the develop- and get along satisfactorily with a broad fin 

 ment of jaws, it became possible for them in the tail region and a dorsal fin to aid in 

 to pursue and capture their prey. Under locomotion. The even more primitive am- 

 these circmnstances a heavy exoskeleton phioxus manages to steer itself with paral- 



so many forms of bone or a hardened bone- 

 like substance. 



THE FIRST APPENDAGES 

 AND JAWS 



The lower forms such as the cyclostomes 



