578 



INVEETEBEATA 



CHAP. 



on each side and passes forwards near the mid-dorsal line to join the 

 groove marking the front border of the collar. In the mid-dorsal line 

 there is consequently a strip of ectoderm uncrossed by the grooves. 



''t.b 



ttr 



Fig. 422. — Illustrating the metaniarpho.si.s of tlie 

 Bahamas Tornaria. (After Morgan.) 



A, Tornaria just before the metamorphosis. B, 

 Tornaria during the metamorphosis. C, Young Balano- 

 glossid worm with tliree pairs of gill-slits, ap, apical 

 plate ; ciLlong, degenerating longitudinal ciliated band ; 

 col, collar region; col.p, collar-pore; ex, excretory tissne 

 in posterior wall of proboscis coelom ; g.p, endodermic 

 gill pouch ; g,s, gill-sht ; pr.p, proboscis-pore ; t.h, tongue- 

 bar dividing the gill-slit ; t.tr, telotroch ; Ur^, accessory 

 telotroch characteristic of the Bahamas larva. 



This is the rudiment of the dorsal neural plate. It becomes depressed 

 beneath the surface, flaps of the adjacent ectoderm, which we may 

 term neural folds, meet over it, and in this way a dorsal neural 

 tube is formed {n.t., Fig. 423). 



