THE PHYLA HEMICHORDATA AND CHORDATA 



There are reasons for believing that the earliest known fishes, the Ostracodermi 

 (Fig. 18.9), lived in fresh water and perhaps originated there. The ocean 

 was subsequently invaded by some of their descendants, which thus became 

 the first marine fishes. The earliest complete skeletons of ostracoderms have 

 been found in Silurian strata, although numerous bony scales from the 

 Ordovician are regarded as fragments of similar animals. Along with primi- 

 tive representatives of the Osteichthycs, the modern types first appeared in 

 the Devonian as primitive sharks, representing the Chondrichthyes. 



The Agnatha, or Jawless Fishes. The only living members of the class 

 Agnatha, the lampreys and hagfishes (Fig. 18.9), are round-mouthed and jaw- 

 less, with a single nostril on the dorsal side of the head, without appendages, 

 with a persistent notochord, and with rudimentary vertebrae. The extinct 

 ostracoderms are also classed as Agnatha; their fossil remains show that they 

 were round-mouthed and jawless and that some, at least, possessed a single 

 nostril. The hags and lampreys of the present time appear to be the lone 



Cephalaspis — 

 an ostracoderm 



Mouth 



Nasal orifice 



Left common 

 gill aperture 



Myxine 



Mucous pits 



Mouth 



'Gill apertures (7) 



Petromyzon 



Fig. 18.9. Ancient and modern Agnatha, or jawless fishes. Fossil remains of the ostracoderm 

 Cephalaspis are among the oldest known vertebrate fossils. Myxine and Petromyzon are char- 

 acteristic genera of existing lampreys and hagfishes, lacking jaws and paired appendages. 

 (From H. V. Neal and H. W. Rand, Chordate Anatomy, copyright 1939 by McGraw-Hill Book 

 Co., Inc., reprinted by permission; Myxine and Petromyzon after B. Dean.) 



555 



