ALTEBNATE GENERATIONS. 233 



CHAPTER XIV. 



ALTERNATE GENERATIONS. 



IF I succeeded in explaining my subject clearly 

 in the last chapter, my readers will have seen 

 that the five Orders of the Echiiioderms are but 

 five expressions of the came idea ; and I will now 

 endeavor to show that the same identity of struc- 

 tural conception prevails also throughout the two 

 other Classes of Radiates, and further, that not 

 only are the Orders within each Class built upon 

 the same plan, but that the three Classes them- 

 selves, Echinoderms, Acalephs, and Polyps, are 

 also based upon one organic formula. 



We will first compare the three Orders of 

 Acalephs, among which the Hydroids stand low- 

 est, the Discophorae next, and the Ctenophorae 

 highest. The fact that these animals have no 

 popular names shows how little they are known. 

 It is true that we hear some of them spoken of as 

 Jelly -Fishes ; but this name is usually applied to 

 the larger Discophore, when it is thrown upon 

 the beach and lies a shapeless mass of gelatinous 

 substance on the sand, or is seen floating on the 



