CHAPTER 22 



Phylum Chordata. 



Amphioxus, 



Tunicates, 



Vertebrates, 



and Others 



HE phylum Chordata includes the verte- 

 brate animals such as the mammals, birds, 

 reptiles, amphibians, fishes, elasmobranchs, 

 cyclostomes, and a number of marine forms 

 that are less well known. All of these animals 

 are characterized by: 



1. A skeletal axis, the notochord at some 

 stage in the life cycle (Fig. 207). 



2. Paired gill slits connecting the pharynx with 

 the exterior at some stage in the life cycle. 

 All chordates up to and including the 

 fishes carry on respiration by means of gills 

 throughout life. In the higher vertebrates, 

 gill slits or traces of them are usually pres- 

 sent only in embryonic or larval stages. In 

 mammals the gill slits never open. 



3. A central nerve cord which contains a 

 cavity or system of cavities; it is dorsal to 

 the digestive tract. 



These chordate characters all appear at some 

 stage in development, and they may persist, 

 change, or disappear in the adult. 



Figure 199 shows some of the funda- 

 mental differences in the body plan of an 

 achordate and a chordate. 



DIVISION OF THE 

 CHORDATA INTO SUBPHYLA 



In many respects the chordates differ 

 widely from one another, and it is cus- 

 tomary to separate them into 4 subphyla: 



1. Hemichordata.* Two classes of wormlike 

 animals: 



a. Enteropneusta. Acorn (tongue) worm, 

 wormlike with many gill slits. 



b. Pterobranchia. Very small chordates 

 with one pair of gills or none. 



* Some zoologists question the presence of a true 

 notochord in the hemichordates, and they would re- 

 move them from the Chordata and place them in an 

 independent invertebrate phylum, but embryology is 

 the crucial thing and can be interpreted differently 

 depending on what are regarded as the really defin- 

 ing features of a notochord. Because the experts do 

 not agree, classification of the hemichordates mus>^ 

 be considered controversial. 



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