AMPHIOXUS. 307 
The fully-formed chordula larva of Amphioxus thus consists of an 
elongated body with a hollow dorsal nerve-tube opening anteriorly by a 
neuropore and posteriorly by the neurenteric canal, or persistent blasto- 
pore, into the archenteron. Below the nerve-tube is a long dorsal 
notochord and below this the spacious archenteron. Laterally, between 
the archenteron and the epiblast lies a row of mesoblastic somites, 
hollow sacs of mesoblast. 
Fig, 221.—LATERAL VIEW OF YOUNG PELAGIC AMPHIOXUS AT COMMENCE- 
MENT OF LARVAL LIFE x 120. (After HarscHEK.) 
Pharynx. 
Neuropore. Mouth. |, 
il 
First 
Pharyngeal Cleft. Notochord. Nerve Cord. Anus. 
ope. 
Note head cavity (with dotted walls) and the thick-walled pre-oral pit in front of pharynx. 
A little before this stage the embryonic period comes to an end and 
the chordula larva is set_free from the egg-membrane, swimming in the 
water by means of the flagella of the epiblast cells. It still, however, 
subsists upon the diffuse yolk-granules scattered throughout the cells. 
Fig. 222.—DIAGRAM OF YOUNG PELAGIC AMPHIOXUS TO SHOW THE 
DIVISIONS OF MESOBLAST AND Cc@&Lom. (After M‘BRIDE.) 
A ae 
Right Head 
Cavity. | J 
ae Myotomes. 
Collar-sac. Perivisceral Ccelom. 
Elongation of the hind end of the body produces a larva much core 
like Amphioxus in shape; at the same time the notochord grows 
forwards to the extreme front end of the body. The neurenteric canal 
closes and the mouth and anus open, the former at first on the left side. 
The mesoblastic somites have grown downwards round the arch- 
enteron and each has divided into a dorsal and a ventral part. The 
