33 



IntroQuction to tlie Vertetrates- 

 Lower Cnoraates ana Fisnes 



Higher and Lower Chordates. The higher chordates are the vertebrates, the 

 most highly developed of all animals — fishes, amphibians, reptiles, birds, and 

 mammals. They are to a certain degree familiar and commonly known as ani- 

 mals. The lower chordates such as the worm-shaped Balanoglossus and the 

 tunicates formed on the same basic plan are unfamiliar, unrecognized as ani- 

 mals, and altogether unsuspected as relatives of the vertebrates (Fig. 33.1). 

 Dozens of tunicates firmly attached to a wharf pile suggest miniature hot water 

 bottles rather than living relatives of man. Yet they have three fundamental 

 characteristics that occur in every chordate including man, and in no other 

 animals. 



Three Unique Characteristics of Chordates 



1 . All have at one time or another a strong flexible notochord that extends 

 through a part or the whole length of the body. In lower chordates, unless lost 

 by retrogressive evolution, it is present throughout life. In higher chordates, 

 it is fully present only during embryonic stages and is replaced by the vertebral 

 column. 



2. The central hollow nerve cord is dorsal to the digestive canal and en- 

 larged at the anterior end as the brain. 



3. Paired gill pouches which open as gill slits, or traces of them are present 

 in the pharynx at some time in the life of all chordates. Up to and including 

 the fishes, gills on the arches between the slits serve for respiration throughout 

 life. In higher vertebrates, gill slits or traces of them are generally present 

 only in larval or embryonic stages. In mammals, the gill slits never open 

 and only in amphibians do they function in breathing. 



The presence or absence of a notochord, and the dorsal or ventral position 



662 



