UROCHORDA. 



to the left atrial involution, but the anus becomes eventually 

 formed by an opening being established between the left atrial 

 involution and the intestine. 



During the above described processes the test remains quite 

 intact, and is not perforated at the oral or the atrial openings. 



The retrogressive metamorphosis of the larva. 



The development of the adult from the larva is, as has 

 already been stated, in the main a retrogressive metamorphosis. 

 The stages in this metamorphosis are diagrammatically shewn 

 in figs. 10 and n. It commences with the attachment of the 

 larva (fig. 10 A) which takes place by one of the three papillae. 

 Simultaneously with the attachment the larval tail undergoes a 

 complete atrophy (fig. 10 

 B), so that nothing is 

 left of it but a mass of 

 fatty cells situated close 

 to the point of the pre- 

 vious insertion of the 

 tail in the trunk. 



The nervous system 

 also undergoes a very 

 rapid retrogressive meta- 

 morphosis ; and the only 

 part of it which persists 

 would seem to be the 

 dilated portion of the 

 spinal cord in the trunk 

 (KupfTer, No. 28). 



The three papillae, in- 

 cluding that serving for 

 attachment, early disappear, and the larva becomes fixed by a 

 growth of the test to foreign objects. 



An opening appears in the test some time after the larva is 

 fixed, leading into the mouth, which then becomes functional. 

 The branchial sack at the same time undergoes important 

 changes. In the larva it is provided with only two ciliated slits, 

 which open into the, at this stage, paired atrial cavity (fig. 10). 



2 2 



FIG. 10. DIAGRAM SHEWING THE MODE OF 

 ATTACHMENT AND SUBSEQUENT RETROGRESSIVE 



METAMORPHOSIS OF A LARVAL ASCIDIAN. (From 



Lankester.) 



