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. 10 A) which takes place by one of the three papillæ. 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 previous insertion of the tail in the trunk. FIG. 10. DIAGRAM SHEWING THE MODE OF ATTACHMENT AND SUBSEQUENT RETROGRESSIVE METAMORPHOSIS OF A LARVAL ASCIDIAN. (From Lankester.) The nervous system also undergoes a very rapid retrogressive 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 papillæ, including 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). 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 branchial sack to which their inner walls attach themselves. Shortly after this the branchial clefts rapidly increase in number[9]. [9] 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.