APPENDIX B 



657 



Subphylum B. The Vertebrates (Vertebrata). Notochord, if persistent, 

 surrounded by cartilage; if not persistent, replaced by a vertebral column 

 made of cartilage or bone. A brain case (cranium) present and, typically, 

 two pairs of appendages (forelimbs and hind-limbs), each arising from 

 several somites and supported by an internal skeleton. Front end of 

 neural tube enlarged into a brain, the remainder forming the spinal cord. 

 Includes the following classes, the first six of which are cold-blooded, the 

 last two warm-blooded: 



Class 1. The Jawless Fishes (Agnatha). Primitive vertebrates with a single 

 median nostril; the mouth an opening without hinged jaws; without paired fins. 

 Includes two subclasses, the ostracoderms and the cyclostomes. 



Subclass 1. The ostracoderms (*Ostracodermata) are the most ancient known 

 vertebrates. They had heavily armored heads and fishlike tails. The group has 

 been described and illustrated (Fig. 27.4) in Chap. XXVII. 



Fig. B.27. Cyclostomes, the most primitive living fishes. Left, lamprey above, hagfish below. 

 Right, much enlarged view of the mouth disk and rasping "teeth" of the lamprey. (Courtesy 

 American Museum of Natural History.) 



Subclass 2. The lampreys and hagfishes (Cyclostomata) are eel-like forms with 

 circular mouths armed with rasping "teeth," numerous gill slits, and a persistent 

 notochord covered with a cartilaginous sheath; they lack bone and scales. The 

 lampreys are world-wide in distribution in fresh and salt waters. They attach 

 themselves to fishes, turtles, etc. by the sucking mouth, and rasp away the flesh, 

 eventually killing the prey. The hagfishes are all marine. They are nocturnal, 

 spending the day buried in the mud of the sea floor at depths of over 2,000 feet; 

 at night they attack fishes, boring their way into the body by means of a special 

 drilling apparatus and cleaning it out so as to leave only a shell. These two types 

 seem to be descendants of different groups of ostracoderms. 



Class 2. The Primitive Jawed Fishes (*Placodermi). The principal types of 

 placoderms have been described and illustrated in Chap. XXVII. 



Class 3. Sharks, Rays, and Chimaeras (Chondrichthyes). These are the car- 

 tilaginous fishes. All surviving types are marine. In this class bone has been lost; 

 the notochord is persistent, though partially replaced by the centra of the verte- 

 brae. The jaws are advanced in structure, and armed with many pointed or 

 flattened teeth. There are numerous gill slits, exposed or covered only by a flap 



