THE CEPHALOCORDA 195 



manner from the archenteric cavity, it is usual to call it an 

 enteroccele. 



The second pair of ccelomic cavities is formed as a pair of 

 outgrowths of the archenteron in front of and rather above the 



lateral grooves. The right outgrowth soon becomes con- 

 stricted off from the archenteron ; the left retains its connec- 

 tion with it for a long time. These cavities have a somewhat 

 complicated history. Their walls give rise to the first myotome, 



