16 



MET A MORPHOSIS. 



BRAIN 



HOTOCHORO 



TAIL 



being established between the left atrial involution and the intes- 

 tine. 



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 11. It 

 commences with the attachment of the larva (fig. 1 A) which takes 

 place by one of the three papillae. Simultaneously with the attach- 

 ment the larval tail under- 



A ., m oes 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 previous inser- 

 tion of the tail in the trunk. 

 The nervous system also 

 undergoes a very rapid re- 

 trogressive metamorphosis ; 

 and the only part of it which 

 persists would seem to be 

 the dilated portion of the 

 spinal cord in the trunk 

 (Kupffer, No. 28). 



The three papillae, in- 

 cluding that serving for at- 

 tachment, 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 bran- 

 chial 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). 



The openings of the atrial cavity at first are shut off from 

 communication with the exterior by the test, but not long after the 

 larva becomes fixed, two perforations are formed in the test, which lead 

 into the openings of the two atrial cavities. At the same time the 

 atrial cavities dilate so as gradually to embrace the whole bi'anchial 

 sack to which their inner walls attach themselves. Shortly after 

 this the branchial clefts rapidly increase in number 1 . 



1 The account of the multiplication of the branchial clefts is taken from Krohn's 

 paper on Phallusia mammillata (No. 24), but there is every reason to think that it holds 

 true in the main for simple Ascidians. 



FIG. 10. DIAGRAM SHEWING THE MODE OF 

 ATTACHMENT AND SUBSEQUENT RETROGRESSIVE 



METAMORPHOSIS OF A LARVAL ASCIDIAN. (From 



Laukester.) 



