390 



THE ANIMAL KINGDOM 



Vertebra- 



Notochord-, 

 Gonadi 



1 



rOviduct 



Myomere - 



Spinal cord 



Spleen 



i>p 



-Ercretory 

 duct 





£2 



Mouth-* 



"Olfactory 

 organ 



Gill alii? 



Bsopha^^ 



'Its 



Heart 

 ■Phsxyroc 



Gall- 

 bladder 



Pancreas 



•-Stomach 

 Intestme- 



Cloaca 

 Urinary bladder 

 Kidney 



•-Coelora 



Figure 20.5. A diagrammatic sagittal section through a generalized vertebrate to 

 show the characteristics of vertebrates and the arrangement of the major organs. 



fishes and are placed in the class Amphibia. Adult frogs, salamanders 

 and other amphibians are more or less terrestrial, but they generally 

 return to the water to reproduce. Amphibians gave rise to the class 

 Reptilia, a group that includes turtles, alligators, lizards and snakes. 

 Reptiles are better adapted to the terrestrial environment and reproduce 

 on land, but they resemble all of the lower vertebrates in being cold- 

 blooded. The remaining two classes, the birds (class Aves) and mam- 

 mals (class Mammalia), evolved from the reptiles, and the members of 

 both groups have become active and warm-blooded. Birds are clothed 

 with feathers and lay eggs; most mammals are covered with hair and 

 give birth to living young which are nourished by milk secreted by the 

 mammary glands. 



Vertebrates share with the lower chordates the three diagnostic 

 characteristics of the phylum. The latter are clearly represented at some 

 stage in the life history of the various groups. A dorsal, tubular nerve 

 cord, which has differentiated into a brain and spinal cord, is present 

 in the embryos and adults of all (Fig. 20.5). Embryonic vertebrates 

 have a notochord lying ventral to the nerve cord and extending from 

 the middle of the brain nearly to the posterior end of the body, but a 

 vertebral column replaces the notochord in most adults. All embryonic 

 vertebrates have a series of pharyngeal pouches that grow out from 

 the lateral walls of the pharynx, but these pouches break through the 

 body surface to form gill slits only in fishes and larval amphibians. 



Vertebrates differ from the lower chordates most obviously in having 

 at least traces of a vertebral column, and in having a better developed 

 head containing an aggregation of sense organs and an enlarged brain 

 enclosed in a brain case or cranium. An alternate name for the sub- 

 phylum, the Craniata, emphasizes this last point. In addition, the 

 superficial layer of vertebrate skin is a stratified epithelium rather than 

 a simple epithelium. A liver, serving as a site for food storage and con- 

 version, is present as a ventral outgrowth from the digestive tract. In 



