296 



.CHORD AT A. 



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 papill(E^ 

 and two lateral pits sink in from the epiblast covering the 

 trunk to form the paired atrium. 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. ) 



Alriopore. Brain with Eye 



Remains of Caudal _ I 



Intestine. Intestine. \^5 



Neuropore. 



Endostyle in Wall 

 of Pharynx. 



Notochord. 



Cord. 



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, i.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 



