338 EMBRYOLOGY. 



507. The type of ABTICTTLAT A, having but one cavity, grow- 

 ing from below upwards, and the nervous system forming a 

 series of ganglions, placed below the intestine, is represented 

 by a single crescent, with the horns directed upwards. 



508. The type of MOLLTTSCA having also but one cavity, 

 the nervous system being a simple ring around the esophagus, 

 with ganglions above and below, from which threads go off 

 to all parts, is represented by a single crescent with the horns 

 turned down. 



509. Finally, the type of RAPIATA, the radiating form of 

 which is seen even in the youngest individuals, is represented 

 by a star. 



