THE ORIGIN OF MAN 39 



Nevertheless, they are a true prophecy of a fish 

 and in them the notochord is well developed as a 

 cylindrical rod of notochordal tissue. However, 

 it is not yet cartiliginous. From the lancelets to 

 the cyclostomes practically all of the interme- 

 diate steps are lost. Similarly, many steps are 

 lost between the cyclostomes and the true fishes. 



All of the fishes and the higher vertebrates are 

 characterized by having articulating jaws, but 

 there are other vertebrates without jaws which 

 are lower in structure. These are known as Cy- 

 clostomata. They are without jaws and paired 

 limbs, and the skeleton is wholly cartilaginous, 

 except the horny teeth, which are set in the mouth 

 in circular and radiating rows. The gills, usually 

 seven in number are in pouches, the external and 

 internal openings being small. 



The seas after Middle Devonian time swarmed 

 with corals, brachiopods, and shellfish, and in gen- 

 eral the life was not very unlike that of the Si- 

 lurian. The coral were widespread, and there 



