AMPHIOXUS. 



307 



The fully-formed chordula larva of Aniphioxus thus consists of an 

 elongated body with a hollow dorsal nerve-tube opening anteriorly by a 

 neuropore and posteriorly by the neurenterie 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. 



ig. 221 



-Lateral View of Young. Pelagic Amphioxus at Commen 

 MENT of Larval Life x 120. (After Hatschek.) 



Pharynx. First 



Neuropore. Mouth. | Pharyngeal Cleft. Notochord. Nerve Cord. 



Anus. 



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 Ccelom. (After M'Bride.) 



ight Head 

 Cavity. 



Left Head 

 Cavity. 



SM^^Su^^^^'^m^^^^^^ 



Collar-sac. 



Myotomes. 

 Perivisceral Coelom. 



Elongation of the hind end of the body produces a larva much more 

 like Amphioxus in shape ; at the same time the notochord grows 

 forwards to the extreme front end of the body. The neurenterie 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 



