296 CHORDATA. 
part of the enteron remains as a mere cord of cells. In 
the trunk the enteron becomes modified into pharynx, 
stomach and intestine and acquires a mouth. The front 
end of the neural tube becomes a hollow brain in which 
are formed a median otocyst and eye. 
At the front end, below the mouth, are formed pafilla, 
and two lateral pits sink in from the epiblast covering the 
trunk to form the paired a¢vium. The anus then opens into 
the left atrium and pharyngeal clefts open into each. Below 
the enteron the heart is formed from mesoblast. Meanwhile 
the tail acquires dorsal and ventral median fins, the noto- 
chordal cells form a strong elastic median axis, and the 
notochord, and mesoblast cells form longitudinal muscles. 
Fig. 210.—TAILED LARVA OF AN ASCIDIAN SEEN FROM 
THE RIGHT SIDE. (Altered from SEELIGER. ) 
Atriopore. Brain with Eye 
and Ear. Neuropore. 
Remains of Caudal . L 
Intestine. Intestine. 
5 
\ 
Papilla, 
Pharyngeal 
Clefts. 
Endostyle in Wall 
of Pharynx. 
In this manner the tail is converted into an efficient loco- 
motor organ by which the larva can move rapidly through 
the water. It is often known as the ascidian tadpole, and 
is evidently a chordate type of comparatively high structure. 
After a period of free life the ascidian tadpole fixes itself 
by its papillae to a rock or other object, and is then con- 
verted into the adult ascidian by a process of retrogressive 
metamorphosis, z.e., a metamorphosis involving simplification 
in structure. 
The sense-organs atrophy, together with the main part of 
the brain and nerve-tube, the notochord and _tail-muscles 
