608 E. W. MACBRIDB. 



series of sensory tentacles such as Cephalodiscus possesses in 

 this region, and which probably correspond to the ambulacral 

 tube-feet or tentacles of Echinoderms. The first is lost in 

 Balanoglossus, owing no doubt to its burrowing life ; but in 

 the free-living Vertebrate ancestor this would not have oc- 

 curred. As the prseoral lobe became reduced in size (a 

 process which may have been connected with the giving up of 

 cilia as a means of progression and obtaining nutriment) the 

 two nervous centres of the Tornaria-like ancestor would be- 

 come approximated, and we should reach the condition which 

 we actually find in the Araphioxus larva, viz. a sense-plate 

 immediately followed by a nervous tube ; for the part of 

 nervous system under the neuropore becomes pigmented, and 

 is sensitive to light. Figs. 27 and 28 are diagrammatic side 

 views of Balanoglossus and an Amphioxus larva, and are 

 intended to emphasise the immense diminution which the prae- 

 oral lobe has undergone in the latter. 



If these conclusions are well founded, Amphioxus would 

 represent a more primitive off'shoot from the Vertebrate stem 

 than Ascidians, for the larvae of the latter possess a large 

 vesicular brain, which only retains a small pore leading into 

 the stomodseum. This deduction is, however, supported by 

 the fact that whereas the Ascidian larva possesses a long post- 

 anal muscular tail (a feature which has become more and more 

 accentuated in fishes), in the Amphioxus larva the anus is as 

 Hatschek pointed out, and as I can confirm, at the extreme 

 posterior end of the body on a vertical neurenteric canal, and 

 becomes only slowly and to a small extent shifted forwards 

 during development. 



Zoological Laboratoey, Cambridge ; 

 August, 1897. 



