AMPHIOXUS 



297 



break up, the tail is resorbed, 

 and the trunk rotates through 

 nearly 180° upon its papillae. 

 In this way the active, sensi- 

 tive, highly - organised " tad- 

 pole" is reduced to a quiescent, 

 sedentary, vegetative ascidian. 

 In Chapter VI. it is ex- 

 plained that ontogeny may in 

 many cases be interpreted as 

 a repetition of phylogeny. 

 This principle applied to the 

 case in hand leads us to the 

 conclusion that the ascidians 

 are descended from active, 

 free-swimming, highly - organ- 

 ised Chordata which have 

 degenerated on the adoption 

 of a sedentary habit. 



Fig. 211. — Transverse Sec- 

 tion THROUGH THE TaIL 



OF AN Ascidian. Larva. 



(After Seeliger.) 



Median Fin. 



Notochord., 



Nerve Tube. 



Muscle. 



Caudal 

 Hypoblast. 



II.— AMPHIOXUS. 



Phylum .... Chordata (p. 403). 

 Sub-Phylum - - - Atriozoa (p. 404). 

 Class Cephalochorda (p. 405). 



Fig. 212. — Lateral View of Amphioxus Lanceolatus x f. 



iAdnat.) 



Myomere Muscles. - 



Dorsal Fin. 



\ 



Tail. I I I 



Anal Fin. Atriopore. Metapleural Fold. 



Amphioxus lanceolatus (the Lancelet) is a small 

 marine organism about one to one-and-a-half inches in 

 length. It is of elongated, fish-like shape, tapering at each 



