508 



Comparative Animal Physiology 



of stimulation, particularly by light and chemical stimulation. When such 

 animals crawl along one side of an object they turn toward the stimulating 

 surface when they reach the end of the object (Fig. 180, A), whereas if they 

 crawl in the crack between two symmetrical objects they continue straight 

 ahead at the end (Fig. 180, B). If, after the animal has given this positive 

 response to an edge (or positive stereotactic response), the stimulating object 

 is withdrawn, the crawling animal then straightens the anterior end to be 

 in line with the posterior end, the homostrophic reflex (Fig. 180, A). These 

 contact reactions depend on unequal stimulation of the two sides and have 

 been demonstrated for many animals, including earthworms, diplopods, in- 

 sect larvae, and voung (blind) rats and mice.^'^- ^" 



A 



B 



Fig. 180. A, Stereotropic orientation of larval Tenehrio (a, h, successive positions) and 

 reflex homostrophic orientation (c), when source of contact is removed. B, Balanced efi^ect 

 of bilateral contact resulting in straight course; removal of contact plate on one side 

 followed by stereotropic bending. From Crozier.^^ 



Many animals show direct reactions to local mechanical stimulation. For 

 example, most fish when stimulated on one side near the posterior end turn 

 the caudal fin toward the side of stimulation. Many sessile animals and 

 worms, such as hvdroids, sea anemones, holothurians, sipunculoids, and an- 

 nelids, withdraw by a strong body contraction when stimulated mechanically 

 at the anterior end. 



Another type of contact orientation is rheotaxis or orientation to a stream 

 of water. Planaria often react positively to a stream, depending on receptors 

 scattered over the body surface.'*'^ Paramecia are positively rheotactic under 

 most conditions. The orientation of fish into a stream depends much more 

 on the eyes than on mechanoreceptors. Blind fish do not orient in a stream 

 unless they touch the bottom, when they orient by frictional stimulation.-" 

 Similarly, xisual cues are impt)rtant in rheotaxis of the lobster, in the elasmo- 

 branch Raja headward flow of fluid in the hyomandibular lateral line canal 



