230 HOMOLOGIES. 
delicate tracery. It is much to be regretted that 
these lower marine animals are not better known. 
The plumage of the tropical birds, the down on 
the most brilliant butterfly’s wing, are not more 
beautiful in coloring than the hues of many 
Radiates, and there is no grace of motion sur- 
passing the movements of some of them in their 
native element. The habit of keeping marine 
animals in tanks is happily growing constantly 
more popular, and before long the beauty of these 
inhabitants of the ocean will be as familiar’ to us 
as that of Birds and Insects. Many of the most 
beautiful among them are, however,. difficult to 
obtain, and not easily kept alive in confinement, 
so that they are not often seen in aquariums. 
Having thus endeavored to sketch each differ- 
ent kind of Echinoderm, let us try to forget them 
all in their individuality, and think only of the 
structural formula that applies equally to each. 
In all, the body has three distinct regions, the 
oral, the ab-oral, and the sides; but by giving a 
predominance to one or the other of these regions, 
a variety of outlines characteristic of the differ- 
ent groups is produced. In all, the parts radiate 
from the oral opening, and join in the ab-oral 
region. In all, this radiation is accompanied by 
rows of suckers following the line of the diverging 
rays. It is always the same structure, but, en- 
dowed with the freedom of life, it is never monot- 
