RAYS AND BILATERAL SYMMETRY 263 



the phenomenon to fall. His statement we quote in full, as it 

 is much to the point in our present discussion : 



" On voit que les Asterides que nous avons consideres comme 

 le groupe terminal des Echinodermes presentent des indices 

 d'une evolution particuliere ; i° I'animal se decentralise pour 

 prendre une apparence radiee (comme les Crinoides et les 

 Ophiures); 2° les divers antimeres formes de trois feuillets, 

 ectoderme, mesoderme, entoderme, sont devenus relativement 

 independants et peuvent en se detachant reproduire un nouvel 

 animal ; 3° le nombre 5 n'a plus sa valeur fatidique : les 

 rayons peuvent etre en nombre quelconque, variable meme chez 

 les individus d'une meme espece ; enfin les rayons peuvent se 

 bifurquer ou meme se trifurquer a leur extremite, ou en un 

 point quelconque de leur trajet, processus qui peuvent aboutir a 

 la formation de nouveaux bras intercales entre les anciens. Ce 

 phenomene est toujours inconstant et irregulier, et n'arrive 

 jamais a la division dichotomique si reguliei»e des bras de Crin- 

 oides ou d'Euryales " (p. 634). 



The inconstancy and irregularity of the phenomena of new 

 ray formation here affirmed, and apparently necessary to such 

 an interpretation of the process, certainly finds no support in 

 what takes place in Pycnopodm, and, as we have shown, the 

 process will probably be found to be perfectly definite in Helt- 

 aster also. 



Concerning the relation of the bilateral symmetry of adult to 

 larva, our observations are of more significance in a general 

 way than for any direct proof which they afford on the ques- 

 tion. 



In view of the opinion of zoologists whose knowledge of echino- 

 derm morphology is so extensive as is Ludwig's, Semon's and 

 Cuenot's, that, in the words of the first mentioned, " Keine 

 derartige Ebene (/. e.^ of bilateral symmetry common to both 

 adult and larva) am Seesterne vorhanden ist und vorhanden 

 sein kann," it may seem futile to hazard even a suggestion to 

 the contrary on the strength of such evidence as we have here. 

 Nevertheless, we cannot help thinking that it would be rather 

 remarkable were an adult animal to possess bilateral symmetry 

 so strongly marked as Pycnopodia does, and yet that this 

 should be wholly independent of the equally strongly expressed 

 bilaterality of its larval stage. 



