32 HEMICHORDATA CHAP. I 
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. 
