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 Coelenterates 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, so they cannot be 

 the exclusive seat of the reaction to gravity. The orienta- 

 tion 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 

 Coelenterates Antennularia (Loeb), Sertulariella (Driesch), 



