CHAP. XVII 



TEOTOCHORDATA 



603 



The changes which occur during metamorphosis have been 

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



f *~: \. - \- x -^- ** *"** - ""^Jyjtrfi^-ip % p=t^ ^_^^j^j/-'- jT-j'' 



p.g.s< 



FIG. 441. Illustrating the metamorphosis of the larva of /////<// iu.>-n?: lanceolatus. 



(After Willey.) 



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

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

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

 six secondaryigill-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, live of which are divided by tongue-bars ; 

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

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

 viewed from the ventral side, nt, atrial cavity ; at.f, atrial fold ; ti.r, buccal cavity ; b.i; brain-vesicle ; 

 i-li, notochord ; cl.ijl, club-shaped gland; en, endst, endostyle; /./.'-, tin-ray cavities; l.h.c, left liead- 

 cavifcy ; m, border of mouth ; M.p, Mtiller's papillae; m.yi./, niefapleural fold; .<, nerve con I ; //./, 



neural tube; nc, eye-spot; 



" 



'-, ;>.;(.'*, the first, second, eleventh, twelfth, 



and fourteenth primary gill-slit, s ; />///, pigment sjmts in nerve cord ; .-.(/..<', 



Ihe liist and 



secondary gill-slits ; slum, stomodai'Uin ; '.''. tongue-bai ; c," vcliini : /'., wheel-organ. 



club-shaped gland disappear simultaneously. Sc\'cra,l of the more 

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



