Chap. 33 



VERTEBRATES LOWER CHORDATES AND FISHES 



663 



PROTOZOA 



Fig. 33.1. A simplified family tree of the animal kingdom suggesting the 

 probable relationships of vertebrates. Studies generally agree that coelenterates, 

 such as jellyfishes, sea-anemones, and corals are the basic stock of all animals 

 above the protozoans and sponges. Clues to any ancestral relationship between 

 invertebrates and vertebrates are still unsatisfactory. Certain similarities between 

 vertebrates and, strangely enough, the echinoderms have been discovered. They 

 are claimed to establish some affinity between the two groups though by no 

 means placing the echinoderms as ancestors of vertebrates. (Reprinted from 

 Man and the Vertebrates by A. S. Romer by permission of The University of 

 Chicago Press. Copyright 1933.) 



of a central nerve cord and heart are invariable differences between verte- 

 brate and invertebrate animals (Fig. 33.4), 



Lower Chordates 



These constitute three subphyla of little -known animals, but they are sig- 

 nificant because of their relationship to echinoderms on one hand and to verte- 

 brates on the other (Fig. 33.2). 



Hemichorda. In the Hemichorda, represented by the acorn worm Bala- 

 noglossus the so-called notochord is a short tubular outgrowth that extends 

 forward from the mouth into the proboscis (Fig. 33.3). It stiffens this muscu- 



