4 PRIMITIVE ANIMALS 



these animals either from the ectodermal or the 

 endodermal cell-layers. 



In animals of the Coelenterate type the symmetry 

 of the body is radiate ; we cannot ordinarily dis- 

 tinguish a dorsal or ventral surface, a right or a left 

 side of the body. The Coelenterates pass their life 

 either fixed to some object from which they stretch 

 out their tentacles in all directions in search of food, 

 or else, as in the case of the jelly-fishes, they float or 

 propel themselves in the water very much at random, 

 being unable to direct their course for any prolonged 

 period in a straight line. The organs of sense and 

 locomotion in these animals are disposed radially, 

 hence the absence of direction in their movements. 



When we pass to the next lowest group of 

 animals, the PlatyJielmia or Flat-worms, we meet 

 with an entirely different type of symmetry, which 

 is functionally correlated with a more advanced and 

 purposive mode of locomotion. The body has 

 ordinarily a flat ventral surface upon which the 

 animal creeps and on which the mouth opens ; the 

 opposite surface is the back or dorsal surface, and 

 the animal is bilaterally symmetrical about its long 

 axis. We can distinguish an anterior and a posterior 

 end ; at the anterior end the brain is situated and 

 usually, in association with the brain, special sense 

 organs such as eyes are present. The achievement of 

 this bilateral symmetry was evidently a fundamental 



