METAMORPHOSIS 6 1 



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 withdrawn from the test and absorbed, and partly cast off in 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 etc., are withdrawn into the body, where they break down and 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 -ganglion 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, or 

 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 

 growth 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 E 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 



