268 



SUBPHYLUM VERTEBRATA: 



birds mammals 



\ / 



repti 



/ 



reptiles 



/ 



lungfishes lobe-fins 



amphibians 



ray-fins choanates 



bony-fishes cartilage fishes 



placoderms 



haqfishes, lampreys ostracoderms 

 archaic cyclostomes 



/ 



lancets filter-feeder vertebrates 



/ 



advanced chordates 



f 



tunicates hemichordotes 



beard worms 9'" filter-feeders ^^^^^ v^/orms 

 .-> \. ■> . 



arm feeders 



oc^ 



t 



pentoctulas 



t 



dipleurulas 



coelenterates 



Figure 16.1 The family tree of the Chordota as reloted to probable 

 evolution of on echinoderm-chordate line. 



Some of these stem reptiles also evolved into the 

 "ruling reptiles," which included dinosaurs, and into 

 the birds and the mammals. 



Subphylum VERTEBRATA (Vertebrates) 



Diagnosis: small to huge animals with a backbone, 

 or vertebral column; chordate characters in develop- 

 mental or larval stages, present or absent in adults. 



FISHES: SUPERCLASS PISCES 



Vertebrates often are separated into two subclasses, 

 fishes and land vertebrates. Such a separation is for 

 convenience, rather than to show relationships. Re- 

 call that the ancestral vertebrates did not give rise to 

 fishes and land vertebrates; the land vertebrates 

 evolved from lobe-finned fishes. 



Superclass PISCES (Fishes) 



Diagnosis: aquatic vertebrates (some are periodi- 

 cally terrestrial); body generally laterally compressed 

 and streamlined, specialized to offer little resistance 

 to water; usually with two sets of paired fins (the 

 evolutionary counterpart of legs) and often with addi- 

 tional unpaired fins; generally with both a body 

 covering of scales and respiration involving gills; with 

 neither a neck region nor a specialized movable joint 

 between the head and the rest of the body. 



CLASS CYCLOSTOMATA (Cyclostomes) 



Diagnosis: body elongate, cylindrical, to about 

 3 feet, without scales; skeleton of gristle (cartilage); 

 median fins on tail and part of body; appendages 

 absent; jaws absent; gills in pouches with external 

 slits, 6 to 14 pairs; fresh-water and marine (Figure 

 16.9). 



ORDER PETROMYZONTOIDEA (Lampreys) 



Diagnosis: mouth ventral, surrounded by a suck- 

 ing funnel-like structure; eyes not covered by skin; 

 seven pairs of gill pouches and seven pairs of external 

 openings start close behind each visible eye; dorsal 

 fin divided; sexes separate, eggs small, larval stage 

 present and of long duration; fresh-water and marine. 



