ALTERNATE PHASES IN FOLLICULINA. 83 



of the everted lip. The making of the lip is one of the most 

 specialized associations of form and secretion in the whole gamut 

 of changes that the animal can run through. As indicated in 

 Figs. 12-13 the head that has been held for more than an hour 

 during the secretion of the spiral tube like a short plug or cork 

 in form and position now suddenly changes into a flat terminal 

 disk which projects outward all round about the mouth of the 

 tube like a mushroom cap from the body of the animal as a stalk. 

 In this mushroom, or disk-phase, of the head the adoral zone 

 becomes excentric on the terminal face of the disk and soon 

 disorganizing changes of the central parts initiate the future 

 completion of dedifferentiations that are to be followed by the 

 differentiation of new arms or feeding organs. But first the 

 overhanging rim of the mushroom normally secretes from its 

 lower surface an annular continuation of the tube which lacks 

 any spiral element and terminates the tube as a horizontal shelf, 

 convex on top; that is, an everted rounded collar comparable 

 to the everted lip of many a potter's jar. However, in this 

 particular instance the collar was never completed, since at I 

 o'clock, August 30, a small planarian imprisoned in the hanging 

 drop came over the incomplete dwelling of the Folliculina and 

 in one minute, thrusting its proboscis into the tube as the 

 Folliculina withdrew, with a few gulps, sucked the entire Follicu- 

 lina up into its digestive cavity. 



But for this unexpected event the Folliculina would probably 

 have soon completed the lip of its dwelling tube and then have 

 differentiated the usual feeding apparatus, with the aid of which 

 it might have accumulated energy and eventually have returned 

 to normal size after successive alternations of form and of 

 activity. 



Comparing the alternations of phases of the normal with those 

 in this individual hampered by restrictions, we observe that the 

 normal was nearly attained, except in the function of secreting a 

 house; and in periods of time closely alike in the two cases. 



There is here remarkable adherence to the normal rhythm 

 despite changed external conditions. When the feeding form 

 could not expand and feed as normal it retrograded into the 

 swimming mouthless phase and this after the usual time of 



