THE PHYLUM CHORDATA 53 



the neural tube and is much more like a vertebrate brain than is that 

 of Amphioxus, since it has connected with it paired optic vesicles and 

 otocysts. The tadpole larva therefore makes up for the deficiencies 

 of the adult in having a good notochord and a neural tube. It is, 

 however, embryonic in the pharynx and ahmentary tract. At first 

 there is no mouth and no anal opening and the pharynx has only a 

 very few clefts. The atrium and atriopore also are at first absent. 



Larval Metamorphosis. — The tadpole larva leads an active free- 

 swimming life for a short time and then settles down upon the bottom 

 or upon some other solid object. Attachment is secured by means of 

 three sucker-like papillae or "chin-warts" and undergoes a very rapid 

 metamorphosis. All of the structures pertaining to a free active life 

 atrophy and the food-concentrating apparatus becomes greatly elabo- 

 rated. Tail, notochord, and neural tube disappear. The brain de- 

 generates to a ganglion. While the tail is atrophying the chin or 

 ventral side of the pharynx grows out of all proportion to the rest of 

 the body so that the mouth is pushed away from the point of attach- 

 ment and comes to be at the free end. The dorsal side of the pharyn- 

 geal region diminishes rapidly also, so as to cause the stomach and 

 intestine to turn upward into a U-shaped body, the anus opening in 

 the same direction as the mouth. The atrium arises as two ectodermal 

 invaginations on right and left sides. These remain separate for 

 some time, but, after they have grown in so as to surround the whole 

 pharynx, the two cavities fuse and lose the bounding wall in the 

 dorsal side, but remain separate at the median ventral line near the 

 endostyle. There is therefore not much resemblance, so far as mode 

 of origin is concerned, between the atrium of a tunicate and that of 

 Amphioxus. The one point in common is that it is lined with ecto- 

 derm. 



The tadpole larva is a very good chordate larva, quite comparable 

 in most respects to a vertebrate larva, such as that of a fish or a frog 

 tadpole; but the adult ascidian is a lowly creature with an organization 

 not much higher than that of a coelenterate or at best, a worm. It is 

 quite evident then that the ascidian presents a typical case of degen- 

 eration accompanying the assumption of sessile life. The well-devel- 

 oped brain of the ascidian larva indicates that the ancestral chordate 

 had a better brain and head than has Amphioxus, and had an organ- 

 ization not far from intermediate between that of the adult Amphioxus 

 and that of the ascidian larva. 



