230 HOMOLOGIES 



delicate tracery. It is much to be regretted that 

 these lowei 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 monofc 



