THE ANIMAL KINGDOM 7 



so that we must admit that the transition from the 

 Coelenterate to the Platyhelminth type, if it ever took 

 place, is unrepresented by any living or fossil animal. 

 We are equally at a loss in trying to trace the 

 origin of the Nemathelminthes or Round-worm 

 Phylum 1 . These are three-layered, bilaterally sym- 

 metrical, unsegmented round worms, many of them 



Fig. 2. Transverse section through a Platyhelminth. al. alimentary 

 canal, end. endoderm, ect. ectoderm, mes. parenchymatous 

 mesoderm. </. reproductive sacs or gonads representing coelom. 

 n. nerve-cords. 



living in decaying vegetation, others as intestinal 

 parasites in man and Vertebrates (Ascaris). They 

 are triploblastic animals like the Platyhelminths, i.e. 

 their tissues are developed from three germ-layers in 

 the embryo, but their internal organs, intestines and 



1 The term Phylum denotes one of the great branches of the 

 animal kingdom (see Appendix A). 



