308 CHORDATA. 
ventral parts unite together to form the continuous ccelom and the 
dorsal parts divide into three portions, the sclerotome, myotome and 
gonotome, which give rise to the connective tissue, myomere muscles 
and the gonads respectively. 
The larva lives for about three months in pelagic water and then moves 
to the sandy bottom. During this period the rows of pharyngeal clefts 
appear as paired apertures, and the atrium arises as a mid-ventral 
ingrowth of epiblast which pushes the ccelom before it and comes to lie 
around the pharynx. It will be noticed that up to the production of the 
chordula larva the development is closely similar to that of Asc¢dia. 
