Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 141 



traveled in different ways and to different places, and made all manner of new 

 relationships. 



Long before any of this occurred, the ancestors of vertebrates had an in- 

 ternal axial support, the notochord, on the dorsal side of the body below the 

 nerve cord and above the digestive tube (Fig. 9.5). Following their ancestors 

 of millions of years past, every individual vertebrate, including man, has a 

 complete notochord at some time during its embryonic life. In amphioxus the 

 notochord persists through life; in the vertebrates it is replaced by cartilaginous 

 or bony vertebrae. The presence of the notochord at some period of life in all 

 vertebrates as well as in their nearer ancestors is the reason for the name of 

 the phylum Chordata, the group to which they all belong. The more limited 

 subphylum Vertebrata includes only the chordates that have vertebrae, lam- 

 preys, fishes, amphibians, reptiles, birds, and mammals, including man. 



The notochord is a slender rod of turgid vacuolated cells held together so 

 tightly within two sheaths that the whole structure is stiffened like a sausage 

 and the substance itself resembles condensed jelly (Fig. 9.5). In mammals, 

 it is soon replaced by bone and cartilage except possibly for a small part of 

 the cartilaginous cushion (intervertebral disc) that persists between the verte- 

 brae. In fishes, remains of it persist through adult life. The conical cavity at 

 each end of a vertebra, familiar to us especially in salmon and tuna fish, was 

 once filled with notochordal cells. 



Vertebrae. A vertebra is a ring of cartilage, in sharks and other lower fishes, 

 or of bone surrounding the nerve cord in higher vertebrates (Fig. 9.6). The 



Dorsal 



Dorsal 



Ventral 



CRAYFISH 



exoskeleton 

 (shell) 



— muscle 



nerve cord 



endoskeleton 

 ( notochord ) 



Ventral 

 AMPHIOXUS 



Fig. 9.5. A characteristic and important difference. Cross sections of an inverte- 

 brate (crayfish) with exoskeleton and ventral nerve cord; and a chordate (amphi- 

 oxus) with endoskeleton and dorsal hollow nerve cord. 



