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 

 make the difference between the Orders. The 

 A ctinoids 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 ot 

 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 tho 



