164 THE HISTOEY OP 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 cal- 

 careous 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 (counterpaits, 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 impoi-tant 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 



