320 G. H. PARKEE 



6. GEOTROPISM 



Corymorpha palma when seen under natural conditions sub- 

 merged in water on the mud-flats is almost always found stand- 

 ing upright. Its buried end is as a rule turned at a consider- 

 able angle to the chief axis of its stalk, which when uninfluenced 

 by tidal currents and the like is almost invariably vertical. 

 The constancy of this position can be tested by the following 

 means. An iron nut the hole of which has been filled with 

 paraffine can be used as a convenient base to which to attach a 

 Corymorpha by pinning the mass of tangled filaments, sand, 

 and the like at the end of its secreted tube. An animal once 

 thus firmly attached can be conveniently put in almost any 

 position. If the nut is placed on the bottom of an aquarium so 

 that the stalk of the Corymorpha is vertical, the polyp will re- 

 main indefinitely in that position. If the nut is turned over on 

 its side so that the stalk of the polyp is horizontal, in an hour or 

 two the polyp will be found to have made a sharp bend whereby 

 it has regained its vertical position. 



If a polyp is allowed in the course of a day to attach itself 

 firmly to a small glass plate, it will be found to rise vertically 

 from the plate. If the plate is now held on edge the polyp soon 

 changes its direction and brings its axis to lie parallel to the 

 plane of the plate with the hydranth uppermost. If the plate is 

 turned upside down and suspended in the aquarium in this posi- 

 tion thus causing the polyp to head vertically downward, in a 

 very short time the polyp will turn to the horizontal and rest 

 there against the underside of the plate. Thus with every turn 

 the polyp endeavors to get as near to a vertical position with its 

 hydranth above as is possible. Negative geotropism is, there- 

 fore, a well marked feature of Corymorpha. 



That the position assumed by Corymorpha is not the result 

 of some such purely mechanical condition as the specific gravity 

 of its parts has already been shown by Torrey ('04 a, p. 404), 

 who demonstrated that the whole polyp is heavier than sea- 

 water and that the hydranth has a higher specific gravity than 

 the stalk. 



