150 



Descriptive Zoology. 



the subject. It was found that the larval ascidian possesses 

 a long tail in which is a distinct notochord and an elongated 

 nerve cord. But early in life the larva attaches itself by 

 its head, the tail gradually disappears, and the elongated 

 nerve cord becomes shortened to a mere ganglion. In the 



Mouth 



Water exit 



Heart 



FIG. 97. DIAGRAM OF A TUNICATE OR ASCIDIAN (SEA SQUIRT). 



From Kingsley's Zoology. 



sessile adult animal no trace remains of the primitive noto- 

 chord. This illustrates what is called " retrograde devel- 

 opment " ; or, in simple words, the ascidian is a degenerate 

 chordate, perhaps even a degenerate vertebrate. This is 

 one instance of degeneration in which the real relationship 

 is indicated by the structure of the young rather than by 

 that of the adult. 



