PHYLUM CHORDATA 



367 



it then settles on the bottom and attaches itself by adhesive papillae 

 located in the anteroventral position. Some authors express it by 

 saying this larva settles on its "chin." It now undergoes regressive 

 changes involving loss of tail, notochord, and posterior portion of 

 nerve cord. The anterior portion of the cord becomes a simple gan- 

 glion. The paired eyes and otocysts (ear structures) also disappear. 

 The dorsal side shortens while the ventral side leug-thens. This places 

 the mouth in a dorsoanterior position, the anus in the dorsoposterior 

 position, and bends the alimentary canal into a U-shape. The num- 



d.n.c. 



trhqa. 

 tnt. / ;e. 



epw. 



^'9-3- cie. 



at.oji 



Fig. 213. — Metamorphosis of an ascidian lari'a. A, larva ready for fixation. 

 B, an intermediate stage of metamorpliosis. G, completion of metamorphosis. 

 ad~ga., adult ganglion ; at., rudiment of atrium ; at.oj}., atrial opening ; ce.ves., 

 cerebral vesicle ; ci.f., ciliarv funnel ; d.n.c, dorsal nerve cord ; e., eye ; eiric, epi- 

 cardium ; est., endostyle ; fix., fixation papillae ; ga., ganglion ; g.s., gill slits ; 

 h,., heart ; int., intestine ; m., mouth ; ncli., notochord ; st., stomach ; stat., statolith ; 

 trk.ga., trunk ganglion. (From Borradaile and Potts, The Invertebrata, published 

 by The Macmillan Company.) 



ber of gill slits increases greatly. The atrial cavity is formed by in- 

 foldings from the exterior on each side which surround the pharynx 

 and meet each other. The external opening of this cavity is the atrial 

 siphon. The outer wall of this newly formed cavity is the mantle. 

 Later the tunic is secreted by the mantle to become a protective, cellu- 

 lose covering. This process of metamorphosis has caused an active 



