266 ALTERNATE GENERATIONS. 
constitute the characters upon which their supe- 
riority or inferiority of structure is based. Here 
the comparison is easily made ; it is simply the 
complication and number of identical parts that 
ynake the difference between the Orders. The 
Actinoids stand lowest from the simple character 
and indefinite increase of these parts; while the 
Halcyonoids, with their eight lobed tentacles, 
corresponding to the same number of internal 
divisions, are placed above them. 
In the name of the division to which they all 
belong we have the key-note to the common 
structure of the three Classes whose Orders we 
have been comparing: they are Radiates. The 
idea of radiation lies at the foundation of all these 
animals, whatever be their form or substance. 
Whether stony, like the Corals, or soft, like the 
Sea-Anemone, or gelatinous and transparent, like 
the Jelly-Fish, or hard and brittle, like the Sea- 
Urchins,— whether round or oblong or cylin- 
drical or stellate, their internal structure always 
obeys this law of radiation. 
Not only is this true in a general way, but the 
comparison may be traced in all the details. 
One may ask how the narrow radiating tubes of 
the Acalephs, traversing the gelatinouS mass of 
the body, can be compared to the wide radiating 
chambers of the Polyp; and yet nothing is more 
simple than to thicken the partitions in the 
