THE BNTBEOPNEUSTA. 7 



(1) Disappearance of ciliated band and apical tuft. 



(2) Body walls have become semi-transparent. 



(3) Animal has changed its habitat, creeping into the upper 

 layer of mud. 



(4) A fixing organ has formed as a ventral posterior sucker. 



(5) Ovring to growth of collar-foldj and constriction of pro- 

 boscis-stalk, the mouth comes to be directed forwards. 



(6) Diflerentiation has occurred in the hypoblast cells, 

 marking out the alimentary canal into three regions, viz. 

 branchial, digestive, and intestinal. 



(7) Notochord is distinctly visible. 



(8) Anal perforation present. 



(9) Second pair of gill-slits arise. 



(10) Opercular fold forms, as a circular thickening. 



From this point onwards the principal changes in external 

 features consist chiefly in an increase in size, and in continual 

 progress towards transparency. This latter is so marked a 

 feature that when three pairs of gill-slits are formed the walls 

 of the body behind the collar, which were originally opaque, 

 become perfectly clear and glassy, so that the internal struc- 

 tures are quite distinguishable. The walls of the proboscis 

 and of the collar never entirely lose their primitive opacity. 

 An attempt is made in figs. 1, 2, and 3 to show this gradual 

 transition. It is presumably due to the consumption of the 

 food particles which in the earlier condition were distributed 

 almost uniformly among the cells of the body. This great 

 transparency is not usual among forms that do not lead a free- 

 swimming existence, and is possibly, so to speak, accidental, 

 and correlated to the rapid growth which now occurs. Nume- 

 rous glandular patches and spots are to be seen in the skin. 

 They are developed chiefly on the proboscis, collar, and sucker, 

 those on the latter being refractive and differing somewhat in 

 appearance from the rest. 



The increase, in size seems to be rapid, but as to the length 

 of time that is taken in passing through subsequent stages I 

 have no record, as the specimens were caught from time to 

 time, and not reared in aquaria. 



