132 THE ANCESTRY OF VERTEBRATES 



Tunicates correspond to those of Amphioxus or to those 

 of Craniates? The anterior end of the notochord evidently 

 cannot help us to answer this question. The restriction 

 of the musculature to the tail of Ascidian-larvae and Ap- 

 pendicularia evidently has engendered a corresponding 

 reduction of the anterior end of the notochord. In the 

 "trunk", the mesodermbands no longer produce muscles but 

 only mesenchyme, and as a consequence the notochord is 

 no longer required here. In Ascidian-larvae (KOWALEWSKY, 

 1871) and newly or unhatched Appendicularia (DELSMAN, 

 1910) it is seen still to reach a little distance into the 

 trunk, though not as 'far as under the neuropore, hs is the 

 case in Amphioxus ; soon afterwards, however, it is exclusively 

 restricted to the tail. In a very early stage, however, VAN 

 Beneden and JULIN (1887, p. 269) remark: „Au debut la 

 plaque medullaire ne depassait guere en avant I'ebauche 

 de la notocorde". 



The distinctly dorsal, not terminal, situation of the 

 neuropore makes us infer that it corresponds to that of 

 Amphioxus and to the mouth of the Invertebrate ancestors. 

 If this be right, the part of the body in front of it may be 

 compared with the praeoral lobe of i4m^A/oxws and Annelids. 

 This name was already used by WiLLEY (1893, p. 328, 

 1894, p. 329) for the pait of the body lying in front of the 

 neuropore and of the mouth. He also compared it with 

 the corresponding structure, the snout, o\ Amphioxus {\%^2, 

 p. 333). just as in Amphioxus (and in many Annelid larvae), 

 the entoderm here is originally in contact with the ectoderm 

 but afterwards a space is left between the two representing 

 a part of the primary body-cavity. Just as in Annelids and, 

 partly at least, in Amphioxus, the mesoderm-cells in this 

 cavity proceed from the two mesodeimic plates of the 

 trunk (while in Amphioxus, the right anterior intestinal 

 diverticulum also provides part of them). We must bear in 

 mind, however, that, in giving the name praeoral lobe to this 

 anterior body region of Ascidian larvae, WiLLEY thinks in 

 the first place of the "praeoral lobe" of Balano^lossus from 

 which in recent times the English speaking zoologists 

 are particularly inclined to derive the Vertebrates. However, 

 he mentions in this connection the Annelids, Molluscs and 

 Arthropods also, to which we of course direct our attention 

 in the first place In either case the position of the adhering 

 papillae in Ascidian larvae must correspond to the animal 



