274 THE PROTOCHORDATA. 



Returning now to Balanoglossus, we have to remark 

 that in the Tornaria larva the central nervous system is 

 represented entirely by the apical plate of the praeoral 

 lobe, the situation of the apical plate corresponding to the 

 anterior tip of the proboscis of the adult. Unlike the 

 Annelids, however, the apical plate of Tornaria does not 

 become replaced after the manner of the Invertebrates 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, it completely disappears after 

 the metamorphosis, having become replaced physiologically 

 by the development of the medullary tube in true Verte- 

 brate fashion from the dorsal ectoderm of the collar-region 

 behind the praeoral lobe.* 



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

 characteristic Invertebrate apical nervous system no longer 

 appears in any stage of development, its physiological func- 

 tion having been once for all assumed by the medullary 

 tube (cerebral vesicle + spinal cord) which lies par excel- 

 lence behind the praeoral lobe (Fig. 131). 



Anterior and Posterior Nenrenteric Canals, and the 

 Position of the Mouth in the Protochordates. 



After the postoral medullary tube had led indirectly to 

 the complete obliteration of the praeoral apical nervous 

 system, and had attained to such a degree of development 

 as we find, for instance, in the Ascidian tadpole, the central 

 canal of the cerebro-spinal nervous system appears to 

 have acquired remarkable relations with the alimentary 

 canal. At both ends of the body connecting ducts be- 



* For a detailed account of the formation of the medullary tube in the col- 

 lar-region of Balanoglossus see MORGAN (Bibliography, Nos. 124 and 125). 



