32 HEMICHORDATA 



than Balanoglossus, as has been pointed out by Morgan.^ It is 

 not impossible that the presence of one pair of gill-slits in 

 Cephalodiscus indicates that this animal diverged from the 

 ancestors of Balanoglossus before the gill-slits were metamerically 

 repeated. 



' J. Morphol. ix. p. 72. 



