30 THE TROPISMS 
ward, but Miss Moore has shown that lack of food, changes 
of temperature and other factors may cause a reversal of 
the response. The infusorian Spirostomum has the peculiar 
trait of attaching itself to the bottom by means of a mucous 
thread at the posterior end and orienting itself in a vertical 
position. 
Among the Celenterates as in the Protozoa there are 
many forms in which geotaxis seems entirely absent. The 
fresh water Hydra will attach itself to the bottom or side 
of an aquarium or hang downward from the surface film 
with apparently equal readiness. Many anemones are 
equally indifferent to their position, but Sagartia, according 
to Torrey, will bend upward if attached to the side of an 
aquarium and slowly migrate to the top. Loeb found that 
if Cerianthus, which lives with the lower part of its body 
buried in the sand, is placed in an inverted position in a test- 
tube the foot will curve downward and the bending will 
gradually continue until the animal finally straightens out 
into an upright position. 
In jellyfish which generally swim with the oral side down- 
ward orientation has been attributed to the statocysts which 
occur on the margin of the umbrella. In Gonionemus, 
however, Murbach has shown that orientation is unimpaired 
after the destruction of all these organs; hence they cannot 
be the exclusive seat of the reaction to gravity. The 
orientation is not one which is passively assumed, for, 
as Murbach has shown, specimens which have been killed 
float with the oral surface upward. 
In the Ctenophores the statocyst which is located at the 
aboral pole of the body is a more essential organ of equilib- 
rium, for as Verworn has shown the removal of this organ 
is followed by loss of orienation to gravity. In some 
Cceelenterates—Antennularia (Loeb), Sertulariella (Driesch), 
