CHAP. XVII 



PROTOCHORDATA 



603 



The changes which occur during metamorphosis have been 

 described in detail by Willey (1891). The first gill-slit and the 



T-T-T-rrT^nj jf i ^ ' ' jj I ' ' i ' ' ^ ' ' ' X? ' I 4--LJ. i i M i "=~2_' n *iJ 

 ?fe:v-y.'- r - : . j'" ~: !~:' x '-^r^ v' ; :^grv-H 



.-^.i. -LJI ....-'. .tvirt'-ini '-Jri'V* uf-f>--^"*<_'*' V,i^HJi--^..^ 



rr ; :"'. , \ \7 * 



FIG. 441. Illustrating the metamorphosis of the larva of Amphioxus lanceolatns. 

 (After Willey.) 



A, larva, with full ninnber 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-slits ; the 

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

 six secondary. 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, at, atrial cavity ; at.f, atrial fold ; b.c, buccal cavity ; b.v, brain-vesicle ; 

 rlt, notochord ; d.gl, club-shaped gland; en, endst, endostyle; f.r.c, fin-ray cavities; l.h.c, left head- 

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

 neural tube; oc, eye-spot; p.g.s 1 , p.g.s 2 , p.f/.s", p.g.sW, p.g.s, the first, second, eleventh, twelfth, 

 and fourteenth primary gill-slits ; pit], pigment spots in nerve cord ; c.y.x 1 , s '-.'/-- s4 ', the first and sixtli 

 secondary gill-slits ; stam, 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 



