404 Harry Beal Torrey. 



Gravity affects both free and fixed organisms, and, so far as we 

 are concerned with it, determines orientation, direction of loco- 

 motion, and direction of growth. In free organisms, orientation 

 may or may not be accompanied by locomotion. Davenport has 

 cited the infusorian Spirostomum as an organism which may 

 belong in the latter category. In this case, orientation is finally 

 due to the action of cilia with which the animal is clothed; if loco- 

 motion is associated with orientation here, it is very slight and 

 inconspicuous. Cerianthus, whose negative geotropism was first 

 considered by Loeb ('91), orients itself by means of muscular 

 action. Though a free organism, it pursues a sedentary habit. 

 The same tendencies are manifested by sand-dwelling anemones. 



Among fixed forms may be considered (i) those which are 

 attached aborally, but are also capable of some degree of loco- 

 motion, such as most of the anemones and the hydroid Corymor- 

 pha; (2) those which are permanently attached, such as most of 

 the hydroids; and with these must be classed plants, especially 

 seedlings. I have recently referred ('04) to the geotropism of the 

 anemone Sagartia davisi, the orientation being accomplished, as 

 in Cerianthus, by muscles. In the discussion of the geotropism 

 of Corymorpha to follow, it will be shown that the orientation of 

 the column is probably accomplished, not by muscles, but by 

 means of growth processes comparable with the growth processes 

 responsible for the orientation of seedlings and, presumably, 

 of geotropic hydroids. 



That the characteristic position of the stem in Corymorpha is 

 not due to a difference in the specific gravities of the distal and 

 proximal regions is apparent when it is seen that not only is the 

 hydroid both proximally and distally heavier than water, but 

 distally it is heavier than it is proximally. If a hydroid is placed 

 in a jar of water after having been slipped out of its proximal 

 investment of perisarc, weighted down as that is by sand clinging 

 to the filaments of the hold-fast, it sinks at once, hydranth first, and 

 lies upon the bottom until the proximal end becomes attached. 

 When this occurs, the stem begins to rise, and in an hour is erect. 



The result is in the end the same whether the hydranth is 

 present or absent, whether the stem is cut so that the proximal 



