P.HEOTROPISNI. This is illustrated by the ac tion of fish in heading up stream and by the slime protozoan in creeping against the current.
STEREOntOrISM. This form of tropism is con tact-irritability, which forces the animal to bring the ventral side of the body in contact with solid bodies. Thus the ventral side of the star fish is positively stereotropic; in other words, the starfish, of the bottom, becomes restless if its ambulacral feet are not in contact with solid If earthworms are placed in a transparent closed vessel they appear to be posi tively stereotropic. As soon, says Loeb, as they reach an angle in the aquarium they remain there, crawling along where the glass can touch them on two sides. This form of tropism plays, says Loeb, a very important part in the processes of pairing and in the formation of organs. The tendency of many animals to creep into cracks and crevices has, he claims, "nothing to do with self-concealment. hut only with the necessity of
bringing the body on every side in contact with solid bodies." The eel is positively stereotropic. It is forced to bring every part of its body, as far as possible, in contact with solid bodies. Every segment of the eel's body is stereotropic, and if touched with the finger on one side there result positively stereotropic curvatures toward the finger. There is in such :u act no more con sciousness than in the boring of a root in the sand (Loeb). Stereotropism is called 'thiginot ropism' by some authors. Another form of trop ism is traumatotropism.
Consult Loeb, Comparative Physiology of the Brain aml Comparative Psychology (New York, 1902) : also Wheeler. "Anemotropism and Other Tropisms in Insects," in Archir fiir Entiniekr lnnqsmechanik der Organismen, vol. viii. (Leip zig, 1S99).