BRANCH CHORD AT A : THE VERTEBRATES, ETC. 261 



Batrachia (batrachians), Reptilia (reptiles), Aves (birds), 

 and Mammalia (mammals), belong to the true vertebrates. 

 These classes will be considered in the five following 

 chapters. 



The remaining three classes include a number of strange 

 marine forms which until recent years were considered as 

 worms, but which are now known to be the nearest living 

 allies of the earliest or primitive vertebrates. The rela- 

 tionship of these forms to early types is manifest, not in 

 the appearance or structure of the adult stage, but only 

 during embryonic or larval stages. 



The ascidians. The sea-squirts, or Ascidians, com- 

 mon on the seashore, compose one class of these primitive 





FIG. m.-^-An ascidian or sea-squirt from the coast of California. (After 

 Jordan and Kellogg.) 



chordate animals. They possess a simple, sac-like body 

 (fig. ill), fastened to the rocks by one end, the other being 



