FOURTH SUB-KINGDOM. 



EADIATA. 



This division includes all those animals whose plan of structure is 

 that of radiation from a centre. The essential form of all Ra- 

 diates is an " organic sphere," of which the main axis is vertical, 

 and around which the primary elements radiate uniformly, — the 

 structure presenting none of that bilateral symmetry which obtains in 

 the three higher sub-kingdoms. There are three natural Classes, ex- 

 hibiting the three different ways in which the radiation is carried out; 

 and they all show the radiation in the distribution of both external and 

 internal organs. Five is the typical number, — nearly all the varied 

 forms repeating it in their arms, jaws, ambulacrse, eyes, etc. Even the 

 cartilaginous framework of the disc of every sucker is regulated by this 

 mystic number ! "The form has its nearest approach to the sphere in 

 the Echinus; it becomes cylindrical in the Hollothuria; stellate in the 

 Star-fish ; bell-shaped in the Acaleph ; and trumpet-shaped in the 

 Polyp. No branch of the Animal Kingdom presents so great a diversity 

 of attitudes as the Radiates." Nor do we elsewhere find an entire group 

 of animals so captivating by the exquisite geometry of their forms, or 

 of such delicate, fascinating colors. The living Radiates number about 

 10,000 species, and are found in all seas, although they increase rapidly 

 in numbers toward the tropics, and offer more forms in the southern 

 than in the northern hemisphere. A single genus, Hydra, inhabit fresh- 

 water. This sub-kingdom has been represented with great richness of 

 forms throughout all periods of geological time, and with a steady in- 

 crease in numbers from the Palaeozoic to the present. 



