SPONGES, FLAT WORMS, AND ROUND WORMS 159 



the egestion of faeces taking place through a single aperture, the 

 mouth. 



The nervous system of flat worms shows a marked advance, 

 especially in concentration, over that of the Coelenterates. The 

 presence of a brain near the end of the body directed forward in 

 moving is what would be expected, since this end receives all 

 sensations first, and nerve cells would be developed in the region 

 of greatest stimulation. 



TABLE V 



THE CHARACTERS OF HYDRA AND PLANARIA CONTRASTED 



CHARACTER 



HYDRA 



PLANARIA 



Symmetry 

 Germ layers 

 Digestive system 



Coelom 

 Excretory system 



Nervous system 

 Muscular system 

 Reproductive system 



Radial 

 Diploblastic 

 Coelenteric cavity 



Absent 

 None 



Network of nerve cells 



Processes of ectoderm 

 and entoderm cells 



No accessory reproduc- 

 tive organs 



Bilateral 

 Triploblastic 



Pharynx and branched 

 intestine 



May be represented by 

 genital sacs 



Complicated system of 

 tubes, ending in flame 

 cells and opening to 

 exterior 



Nerve cells concentrated 

 into brain and nerve 

 cords 



Muscle fibers with no 

 other function, from 

 mesoderm cells 



A complicated reproduc- 

 tive apparatus 



