ASTEKIAS PALLIDA. . -^^ 



one that of the pore-canal, and the other that of the stone-canal, as re- 

 presented in Fig. 25. These two openings \v(juld become more and more 

 separated from each other, as the formation of the stone-canal proceeds. 

 It is also evident from the above description that one side of the wall 

 of the inner opening of tlie pore-canal is directly continued into the 



stone-canal. 



From the above description of the first rudiment of the stone- 

 can;d, it is perfectly clear that, it and the pore-canal are two quite 

 distinct structures, there l)eing no trace of a stone-canal in stage 1Î. 

 This fact has been generally recognized by students of asterid develop- 

 ment *[ßury '87 & '9<S, MacBride '96, p. o51], and has received a just 

 recognition in Lang's Lehrbuch ; but as it is not so generally known I 

 think it necessary to emphasize it here once more. No less an authority 

 than Ludwig has remarked long ago ['82, p. 84), " Man hat sich daran 

 ^rewcihiit den llückeni)orus der Echinodermenlarven als die primäre 

 Kommunikations()ttnung der Aussenwelt mit dem Wassergefäss- 

 systeme zu betrachten. Diese Autfassung halte ich nicht für ganz zutref- 

 fend. Man hat dabei einen Umstand nicht in Betracht gezogen, der 



eine grössere Beachtung verdiente— ich meine den limstand, dass 



der Riickenporus anfänglich niemals nur mit dem Hydrocoel in Yerbin- 

 duno- steht, sondern in einen Kaum führt, der zugleich die linke Hälfte 

 des Enterocoels oder das gesammte Enterocœl repräsentirt. Mir scheint 

 es demnach den Thatsachen besser zu entsprechen, wenn man den 

 Riickenporus in seiner primären Bedeutung als einen in das Enterocœl 

 führenden Porus betrachtet, welcher erst sekundär in engere lîeziehung 

 zu dem von dem Enterocœl abgespaltenen Wassergefässsysteme tritt." 

 This opinion has been fully confirmed by the researches of l^>ury and 

 MacBride, and the results of my own observations are such an exact 



* Field ['93, p. 120], however, does not seem to have been aware of the formation of the 

 stone-canal independently of the pore-canal. 



