88 PRIMITIVE ANIMALS 



the suboesophageal ganglion, to shift forwards and 

 to take on the sensory functions of the prostomium. 

 We are therefore far from dismissing as absurd the 

 Annelid theory of Vertebrate descent, despite the 

 morphological difficulty of the apparent change of 

 the dorsal into the ventral surface thereby in- 

 volved. 



There is however another, and possibly more 

 serious, difficulty in the way of its acceptance. 



The peculiar worm-like burrowing animal known 

 as Balanoglossus has for long been associated with 

 the Chordata on account of a number of structures 

 which it possesses and which point to a fairly close 

 relationship with Amphioxus and so with the Verte- 

 brates. In the first place this animal possesses gill- 

 slits of an almost identical structure with those of 

 Amphioxus ; an organ is also present which suggests 

 a notochord, and the arrangement of the coelom 

 has been compared with that found in Amphioxus. 

 Traces of a dorsal nervous system, of tubular structure 

 as in Vertebrates, are present, and the arrangement 

 of the gill-slits suggests the appearance of metameric 

 segmentation. 



Now this animal, as we have seen in the last 

 chapter, pp. 72, 73, passes through a larval stage, the 

 Tornaria, which in all its features recalls the exact 

 structure of the Echinoderm larva, the Auricularia. 

 If we admit the close relationship of Balanoglossus 



