HELIOTROPISM 



63 



4. HELIOTROPISM OF SESSILE ANIMALS 



When we study the effects of light on sessile animals 

 we find that they behave in a similar manner to sessile 

 plants. When illuminated from one side they bend their 

 heads to the source of light until their axis of symmetry 

 goes through the source of light. In this case the sym- 

 metrical photosensitive elements receive equal illumina- 

 tion and the symmetrical muscles are under equal tension. 

 Hence the animal remains in this orientation. These 

 sessile animals were the first examples by which the 



FIG. 24. Tube worms in aquarium, all bending toward light. 



muscle tension theory of animal heliotropism was 

 proved. 288 



Spirographis spallanzani (Fig. 24) is a marine annelid 

 from 10 cm. to 20 cm. long, which lives in a rather rigid 

 yet flexible tube. The latter is formed by a secretion 

 from glands at the surface of the animal. The tube is 

 attached by the animal with its lower end to some solid 

 body, while the other end projects into the water. The 

 worm lives in the tube and only the gills, which are 

 arranged in a spiral at the head end of the worm, project 

 from the tube. The gills, however, are quickly retracted, 

 and the worm withdraws into the tube when touched or 

 if a shadow is cast upon it. 



