ALTERNATE GENERATIONS. 233 
CHAPTER XIV. 
* 
ALTERNATE GENERATIONS. 
Ir I succeeded in explaining my subject clearly 
in the last chapter, my readers will have seen 
that the five Orders of the Echinoderms are but 
five expressions of the came idea ; and I will n®w 
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 Discophore next, and the Ctenophore 
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 
