AMPHIOX US 207 
break up, the tail is resorbed, Fig. 211.—TRANsveRsE Strc- 
and the trunk rotates through TION THROUGH THE TAIL 
nearly 180° upon its papillee. OF AN ASCIDIAN Larva. 
In this way the active, sensi- (anereaivecey 
tive, highly- organised “ tad- 
pole” is reduced to a quiescent, 
sedentary, vegetative ascidian. 
In Chapter VI. it is ex- 
plained that ontogeny may in \ Nerve Lube. 
many cases be interpreted as 
a repetition of phylogeny. 
This principle applied to the ( 
case in hand leads us to the — ypotochord. 
conclusion that the ascidians 
are descended from active, 
free-swimming, highly - organ- 
ised Chordata which have 
degenerated on the adoption 
of a sedentary habit. 
Median Fin. 
‘Caudal 
Hypoblast. 
IL—AMPHIOXUS. 
PHYLUM CHORDATA (p. 403). 
SuB-PHYLUM ATRIOZOA (p. 404). 
Ciass CEPHALOCHORDA (p. 405). 
Fig. 212,—LATERAL VIEW OF AMPHIOXUS LANCEOLATUS x 3. 
(Ad nat.) 
Myomere Muscles. Dorsal Fin. 
Tail. 
“EID [WI 
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 
