ASCIDIA. vil 
in the epithelium lining the cerebral vesicle—they are 
myelonic sense-organs. 
After a few hours, or at most a day or so, the larva 
attaches itself by one or more of the three anterior ecto- 
dermal glandular papille (one dorsal and two lateral) 
to some foreign body, and commences the retrogressive 
metamorphosis which leads to the adult state. The 
adhering papille having performed their function begin to 
atrophy, and their place is taken by the rapidly increasing 
test. The tail, which at first vibrates rapidly, is partly 
withdrawn from the test and absorbed and partly cast off 
in shreds (figs. 10 to 12). The notochord, nerve tube, 
muscles, &c., are withdrawn into the body, where they 
break down and are absorbed by phagocytes, or dissolved 
in the fluid of the body-cavity. The posterior part of the 
nerve tube and its anterior vesicle with the large sense 
organs disappear, and the middle part undergoes prolifera- 
tion dorsally to form the relatively small ganglion of the 
adult, underneath which the neural tube gives rise to the 
hypophysial gland. While the locomotory, nervous, and 
sensory organs are thus disappearing or being reduced, 
the alimentary canal and reproductive viscera are growing 
larger. The branchial sac enlarges, its walls become 
penetrated by blood channels and grow out to form bars 
and papille, and the number of openings greatly increases 
by the primary gill slits becoming broken up into the 
transverse rows of stigmata. 
The stomach and intestine, which developed as an out- 
growth from the back of the branchial sac at the dorsal 
edge, become longer and curve so that the end of the 
intestine acquires an opening into at first the left-hand 
side, and eventually the cloacal or median part of the atrial 
cavity. ‘The adhering papille have now disappeared, and 
are replaced functionally by a growth of the test over 
