REGULATION OF HARENACTIS ATTENUATA. 53 



no appreciable degree ot atrophy within four and one half months. 

 Discussion is unnecessary. 



In a number of cases the lateral incision was made somewhat 

 obliquely so that one end of it was distal to the level of the ostia, 

 the other proximal. In these cases the part in which ostia were 

 present became distended after healing, produced tentacles almost 

 or quite as long as the others and persisted during four and one 

 half months : the part from which ostia were absent became 

 slightly distended after healing, produced short tentacles, then 

 underwent gradual collapse and complete atrophy within a few 

 weeks. Consequently these animals in their final condition 

 possessed a structure arising from one end of the lateral incision 

 and ending free, since the part which originally connected it with 

 the other end of the incision had atrophied, and bearing tentacles 

 on its oral and aboral surfaces. In certain respects these cases 

 are the most interesting of all, since the two different results can 

 be observed in a single animal. 



To sum up : it is possible to determine experimentally whether 

 persistence or complete atrophy of certain parts of the body of 

 Harenactis shall occur : the factor determining the result is the 

 distension of the enteric cavity with fluid or the absence of such 

 -distension. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 December, 1908. 



