ae METAMORPHOSIS 61 
by one or more of the three anterior ectodermal glandular 
papillae (one dorsal and two lateral) to some foreign body, and 
commences the retrogressive metamorphosis which leads to the 
adult state. The adhering papillae, 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withdrawn from the test and absorbed, and partly cast off in 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 (Fig. 26, B, C, D). The notochord, nerve-tube, muscles, 
etc., are withdrawn into the body, where they break down and 
are absorbed by phagocytes. The posterior part of the nerve 
cord and its anterior end with the large sense-organs disappear, 
and the middle part or trunk-ganghon is reduced to form 
the relatively small ganglion of the adult, underneath which 
the hypophysial tube gives rise to the neural gland. While the 
locomotory, nervous and sensory organs are thus disappearing, 
being reduced, the alimentary canal and reproductive viscera are 
growing largely. The branchial sac enlarges, its walls become 
penetrated by blood-channels, and grow out to form bars and 
papillae, and the number of openings greatly increases by the 
primary gill-slits being 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 right side, 
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 
papillae have now disappeared, and are replaced functionally by a 
erowth of the test over neighbouring objects; and at the same 
time the region of the body between the point of fixation and the 
mouth (branchial aperture) increases rapidly in extent, so as to 
cause the body of the Ascidian to rotate through about 180°, and 
thus the branchial siphon is carried to the opposite end from the 
area of attachment (see Fig. 26, B,C, D,E). Finally the gonads 
and their ducts form in the mesoderm between the stomach and 
intestine. We thus reach the sedentary degenerate fixed adult 
Ascidian with little or no trace of the Chordate characteristics so 
marked in the earlier larval stage (see K and A, Fig. 26). The 
free-swimming tailed larva shows the Ascidian at the highest 
level of its organisation, and is the stage that indicates the 
genetic relationship of the Tunicata with the Vertebrata. 
In some Ascidians with more food-yolk in the egg, or in which 
oo? 
or 
