331 



In the Tornaria larva of Balanoglossus the central nervous system 

 is represented entirely by the apical plate of the praeoral lobe. 

 Unlike the Annelids however, the apical plate of Tornaria does not 

 become replaced by the development of a cerebral ganglion arising like 

 it from the ectoderm of the praeoral lobe and with it as a formative 

 centre. On the contrary, the apical plate of Tornaria, as is well-known, 

 completely disappears during the metamorphosis and becomes replaced 

 physiologically by the development of the medullary tube, behind 

 the praeoral lobe, from the dorsal ectoderm of the collar-region. 



In the Ascidian larva however, and in Amphioxus, the characteristic 

 Invertebrate apical nervous system no longer appears in any stage 

 of development, its function having been once for all assumed by the 

 medullary tube which lies emphatically behind the praeoral lobe. 



Macbride 1 ) has made the interesting observation that the remar- 

 kable praeoral lobe of the larva of Asterina gibbosa, besides 

 serving as a creeping organ, becomes converted into an organ of 

 fixation by which the larva attaches itself to the substratum 

 during the metamorphosis. 



This discovery, coupled with what is known of the development 

 of Antedon, shows that the bilateral ancestor of the Echinoderms 

 must have possessed a praeoral lobe which could be used as an 

 organ of fixation. 



It is well-known that in the typical Echinoderm larva there is no 

 apical plate in the praeoral lobe ; — but the transitory ectodermal thicke- 

 ning at the apical pole which has been observed in larvae of Asterids 

 and Echinids; and the well-developed apical plate which actually occurs 

 in the free-swimming larva of Antedon 2 ), show beyond question that in 

 the vast majority of existing Echinoderms the praeoral lobe has secon- 

 darily and independently acquired emancipation from the central nervous 

 system in consequence of being used originally, and in some cases 

 (Asterina, Antedon) still, as the organ of fixation. 



Thus, on the above principle of the gradual emancipation of the 

 praeoral lobe from the central nervous system, having taken place 

 correlatively with change of function of the lobe itself (e. g. con- 



1) E. W. Macbride, „The Organogeny of Asterina gibbosa." Proc. 

 Roy. Soc, Vol. 54, p. 431—436, Sept. 1893. 



2) cf. Seeliger, Studien zur Entwickelungsgeschichte der Crinoiden. 

 Zool. Jahrbücher, Abt. f. Anat., Bd. VI, Heft 2, 1892. 



23* 



