35 



in the epitbeliuin lining tiie cerebral vesicle — they are 

 my clonic 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 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 

 atroph}^ 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 withdrawn from the test and absorbed and partly cast off 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 muscles, &c., are withdrawn into the body, where they 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 papillae, 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 papillae have now disappeared, and 

 are replaced functionally by a growth of the test over 



