THE PHYLA HEMICHORDATA AND CHORDATA 



walls of the gonads. The development of the amphioxus has been described 

 in Chapter 5 (p. 147). 



The chordate features of Amphioxi are readily apparent from the fore- 

 going description. The dorsal, tubular central nervous system is formed from 

 the dorsal ectoderm. Fhe notochord is obviously comparable with that of the 

 vertebrates, both in its structure and in its anatomical relationships to the 

 nerve cord and the gut. The gill slits, the arrangement of the blood vessels, 

 and the course of the blood fiow, along with other resemblances, mark these 

 animals as chordates, although they are much simpler in their organization 

 than the most primitive fishes. 



The Acraniata thus include two groups, Urochordata and Cephalochordata, 

 which are evidently closely related, although they are widely divergent 

 types. Evidence from comparative embryology and comparative anatomy 

 indicates that these forms are also related to Vertebrata. The evolutionary 

 changes through which the ancestral chordate stem produced these modern 

 representatives must have occurred long before the Cambrian period, which 

 explains the absence of a fossil record to document the steps or stages. 

 Thus, in attempting to establish the relationships between Acraniata and 

 Craniata, we must rely on embryological and anatomical evidence. 



THE CRANIATA OR VERTEBRATA 



A skull, or cranium, and vertebrae are the features that have given to these 

 animals the names Craniata and Vertebrata, respectively. The skull encloses 

 a brain, formed by the differentiation of the anterior end of the neural tube. 

 Correlated with the development of the brain and its skeletal encasement is 

 the fact that the notochord does not extend to the very anterior end of the 

 body, as it does in the amphioxus. In all vertebrates the notochord appears 

 during embryonic development. In many of the lower vertebrates it persists 

 in the adult as a cord running through the center of each vertebra, but in 



Aortic arches 

 Brain 



Spinal cord ^°/^^^ ^o''*^ 



,'Notochord 



Digestive tract 



Gill slits 



Heart 



cavity 



Fig. 18.7. Diasjram of a s;eneralized vertebrate, showing relationships of the chief internal 

 structural features. Lungs, present in a great many vertebrates, and reproductive and ex- 

 cretory systems, present in all, are not indicated in this figure. 



553 



