WORMS AND CCELENTERATA 263 



the roots are so fine that they lie entirely beyond our 

 range of vision. 



The protozoa represent the point at which the root 

 becomes thick enough to be seen by us. These are 

 the simplest organisms known to us, and their origin 

 goes back to an incalculably distant period. From them 

 were developed the ccelenterata, which we will consider 

 at the close of this chapter. After these come the platodes 

 animals that approach closely to the ccelenterates in 

 structure ; in both, for instance, there is only one aperture 

 in the body, and this has to act both as mouth and anus. 



From the platodes (or "flat worms") descended the 

 round worms, which have mouth and anus, and an 

 alimentary canal suspended in a spacious body-cavity. 

 However, their organisation is still very simple. Their 

 type of structure is retained fundamentally in all the 

 higher animals, as they are the ancestors of the verte- 

 brates on the one side, and the molluscs and articulates 

 on the other. We may mention as a fourth daughter- 

 stem the echinoderms, the star-fish and sea-urchins, of 

 which there are many species in the sea. 



It was a fortunate chance for science that the 

 transitional forms from the vermalians to each of the 

 four stems are still in existence. Where they are 

 missing, the gap is filled by larvae forms. Thus the 

 larva of certain molluscs and of the echinoderms 

 resembles a certain order of worms, the microscopic 

 rotifers that are found in all water. The lowest forms 

 of the vertebrates lead on to a group of animals called 

 the tunicates, which have a good deal of affinity to the 

 worms in their structure. 



