1 68 THE HISTORY OF CEEATION. 



anatomy, and by the ontogeny of some Star-fislies (Co- 

 lastra), and of segmented worms. The many-jointed Ring- 

 worms (Annelida) in their inner structure are closely 

 allied to the individual arms or radii of the Star-fishes, 

 that is to the original single worms, which each arm 

 represents. Each of the five worms of the Star-fish is 

 a chain composed of a great number of equi-formal mem- 

 bers, or metamera, lying one beliind the other, like 

 every segmented Worm, and every Arthropod. As in 

 the latter a central nervous cord, the ventral nerve cord, 

 runs along the central line of the ventral wall of each seg- 

 ment. On each metameron there is a pair of non-jointed 

 feet, and besides these, in most cases, one or more hard 

 thorns or bristles similar to those of many Ring-worms. 

 A detached arm of a Star-fish can lead an independent life, 

 and can then, by the radially-directed growth of buds at 

 one end, again become a complete star. 



The most important proofs, however, of the truth of 

 my hypothesis are furnished by the ontogeny or the 

 individual development of the Echinoderma. The most 

 remarkable facts of this ontogeny were 'first discovered 

 in the year 1848 by the great zoologist, Johannes Mllller 

 of Berlin. Some of its most important stages are repre- 

 sented on Plates VIII. and IX, (Compare their explanation 

 .in the Appendix.) Fig. A on Plate IX, shows us a com- 

 mon Sea-star (Uraster), Fig. B, a Sea-lily (Comatula), 

 Fig, C, a Sea-urchin (Echinus), and Fig. D, a Sea-cucumber 

 (Synapta). In spite of the extraordinary difference of 

 forai manifested by these four representatives of the differ- 

 ent classes of Star-fishes, yet the beginning of their develop- 

 ment is identical in all cases. Out of the egg an animal-form 



