164 THE HISTORY OF CREATION, 
The water is pressed from the tubes into a number of 
little hollow feet, which thereby become widely distended, 
and are then employed for walking and suction. The 
Sea-stars are moreover characterized by a peculiar ecal- 
eareous formation in the skin, which in most cases forms 
a firm, well-closed coat of mail, composed of a number of 
plates. In almost all Echinoderma the body consists of 
five radii (counterparts, or antimera) standing round the 
main axis of the body, where they meet. It is only in some 
species of Sea-stars that the number of these radii amount 
to more than five—to 6—9, 10—12, or even to 20—40; 
and in this case the number of radii is generally not constant, 
but varies in different individuals of one species. 
The historical development and the pedigree of the 
Echinoderma are completely revealed to us by their 
numerous and, in most cases, excellently preserved fossil 
remains, by their very remarkable individual develop- 
mental history, and by their interesting comparative ana- 
tomy ; this is the case with no other tribe of animals, even 
the Vertebrata themselves are not to be excepted. By a 
critical use of those three archives, and by a careful com- 
parison of the results derived from their study, we obtain 
the following genealogy of the Star-fishes, which I have 
already published in my General Morphology (vol. ii 
Plate IV. pp. 62-77.) 
The most ancient and original group of the Star-fishes, 
the primary form of the whole phylum, consists of the class 
of the true Sea-stars (Asterida). This is established by 
numerous and important arguments in anatomy and the 
history of development, but above all by the irregular and 
varying number of the radii, or antimera, which in all other 
