CHAP. XVII 



PEOTOCHOBDATA 



603 



The changes which occur during metamorphosis have been 



described in detail by Willey (1891). 



The first gill-sht and the 



^g-^^ pgs^ clgl "^P/ehdst 



Fig. 



141. — Illustrating tliu iiiutiiniorphoais ol' the larva oi^ Amphioxus lanceolaius. 

 (After V\^illey.) 



A, larva, with fall number of primary gill-slits, seen from the right side ; six thickenings of the 

 pharyngeal wall above the primary gill-slits indicate the beginnings of the secondary gill-shts ; the 

 atrial folds are not yet united. B, larva in which the hindermost primary gill-slits have disappeared ; 

 six secondary 1 gill-slits have been formed and the atrial folds are united ; viewed from the right side. 

 C, larva in which eight secondary gill-slits have been formed, five of which are divided by tongue-bars ; 

 the primary gill-slits have been reduced to nine, and tongue-bars have begun to be formed in them ; 

 the endostyle has been shifted to the mid-ventral line, and oral cirri are beginning to be formed ; 

 viewed from the ventral side, ai, atrial cavity ; at.f, atrial fold ; Zj.c, buccal cavity ; &.v, brain-vesicle ; 

 ch, notochord ; cl.gl, club-shaped gland ; en, endst, endostyle ; f.r.c, fln-ray cavities ; Lh.c, left head- 

 cavity ; m, border of mouth ; M.p, Mliller's papillae ; m.p,/, metapleural fold ; n.c, nerve cord ; n.t, 

 neural tube; oc, eye-spot; p.g.s^, P-0'S^, P-3-s^^i P-O-^^^, P-U-s^^, the first, second, eleventh, twelfth, 

 and fourteenth primary gill-slits ; pig, pigment spots in nerve cord; s.g.s\ s.g.^, the first and sixth 

 secondary gill-slits ; stom, stomodaeum ; t.b, tongue-bar ; v, velum ; w.o, wheel-organ. 



club-shaped gland disappear simultaneously. Several of the more 

 posterior of the " primary " gill-slits also disappear, and in this way 



