DEVELOPMENT OF AMPHIOXUS 527 



The cavity of the gastrula — the archenteron — becomes 

 the gut of the adult, and gives rise to the coelomic pouches. 



The notochord arises along the mid-dorsal line of the 

 archenteron ; its forward extension is secondary. 



During the early part of larval life the ectoderm cells, 

 including those forming the medullary canal, are ciliated. 

 At this stage the larva is much more active than the adult. 



The development of the mesoderm and the coelomic 

 pouches has already been explained (Fig. 298). The freely 

 swimming larva rapidly elongates by the addition of new 

 myotomes. The front of the gut enlarges to form a 

 pharynx, and a mouth breaks through on the left side. 

 The anterior end becomes pointed and the notochord 



Fig. 302. — Larval Amphioxus, from the right side. — After Willey. 



N., Nerve-cord ; E., eye-spot ; NCH., notochord ; V., velum ; • 

 EN., endostyle ; G.S.i, primary gill-slits, above which are 

 developing the secondary gill-slits (G.S.2). 



grows forward rapidly past the nerve-cord. The endo- 

 style appears on the right-hand wall of the pharynx. 

 Fourteen gill-slits break through in the mid-ventral line 

 and then grow well up on the right side of the body, so 

 that the larva has an unpaired row of gill-slits on the right 

 side — the opposite side to the large mouth. 



After three months pelagic life this curious asymmetrical 

 larva sinks to the bottom, and a remarkable metamorphosis 

 takes place. The development of the atrial chamber has 

 been explained (Fig. 297). Most puzzling is the appearance 

 of eight more gill-slits on the right side, forming a row 

 above the first series. The latter are finally forced round 

 to the left side, six of them disappearing on the way, giving 

 two rows of, at first, eight gill-slits. Mouth and endostyle 

 move from their respective positions to the mid-ventral 



