122 



DYNAMICS OF LIVING MATTER 



We find heliotropic curvatures in animals where there can be no 

 doubt that the curvature is due solely to a process of contraction, and 

 not to a process of growth. Spirographis Spallanzani is a marine Anne- 

 lid 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, how- 

 ever, are quickly retracted, and the worm withdraws into the tube when 

 touched or if a shadow is cast upon it. 



When such tubes with their inhabitants are put into an aquarium 

 which receives light from one side only, it requires, as a rule, a day or 

 more until the foot end of the tube is again attached to the bottom of 

 the aquarium. As soon as this occurs, the anterior end of the tube is 



raised by the worm 

 until the axis of sym- 

 metry of the gills falls 

 into the direction of 

 the rays of light which 

 enter through the win- 

 dow into the aquarium 

 (Fig. 22).* When 

 the animal has once 

 reached this position it 

 retains it as long as 

 the position of the aquarium and the direction of the rays of light 

 remain unchanged. If, however, at any time the aquarium is turned 

 180 so that the light falls in from the opposite direction, the animal 

 bends its tube during the next twenty-four or forty-eight hours in 

 such a way that the axis of symmetry of its circle of gill? is again in 

 the direction of the rays of light (see Fig. 23). When the light 

 strikes the aquarium from above, the animals assume an erect 

 position, like the positively heliotropic stems of plants when they grow 

 in the open. 



In these phenomena the mechanical properties of the tube play a 

 role. When the animal is taken out of the bent tube, the latter retains 

 its form. How does this permanent change of form of the tube come 

 about? In my opinion through new layers being secreted on the in- 

 side. The youngest layers of the secretion are more elastic than the 

 old layers, and, moreover, have at first a powerful tendency to shorten. 



* Loeb, Pfiuger's Archiv, Vol. 47, p. 391, 1890. 



FIG. 22. Positive heliotropism of Spirographis Spallanzani. 

 (From nature.) 



